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PubMed1990

Regulation of the glyoxylate bypass operon: cloning and characterization of iclR.

In Escherichia coli, expression of the glyoxylate bypass operon appears to be controlled, in part, by the product of iclR+. Mutations in iclR have been found to yield constitutive expression of this operon, suggesting that iclR+ encodes a repressor protein. We have cloned iclR+ by taking advantage of its tight genetic linkage with the glyoxylate bypass operon. The clone complemented a mutant allele of iclR in trans, restoring an inducible phenotype for this operon. Deletion analysis identified a region of ca. 900 base pairs that was necessary and sufficient for complementation. The nucleotide sequence of the insert was then determined. Translation of this sequence revealed an open reading frame capable of encoding a protein with Mr 29,741 preceded by a potential Shine-Dalgarno ribosome-binding site. The deduced amino acid sequence includes a region at the amino terminus that may form a helix-turn-helix motif, a structure found in many DNA-binding domains.

Amino Acid Sequence↗

PubMed1990

Direct involvement of IS26 in an antibiotic resistance operon.

The plasmid pBWH77, originally found in an isolate of Klebsiella pneumoniae, harbors a new antibiotic resistance operon containing two resistance genes transcribed from an IS26-hybrid promoter, as shown by nucleotide sequencing, mRNA mapping, and the effect of inserting a transcription terminator within the promoter-proximal gene. The nucleotide sequence of this region revealed that the operon (IAB) is made up of three sections that are closely related to previously described genetic elements. The -35 region of the promoter, together with the adjacent sequence, is identical to sequences of the IS26 element. One of the resistance genes, aphA7, which is located next to the hybrid promoter, confers assistance to neomycin and structurally related aminoglycosides. This aphA7 gene is highly homologous to aphA1 of Tn903, with five nucleotide differences. The second gene, blaS2A, encodes an evolved SHV-type beta-lactamase with a pI of 7.6 that confers resistance to the broad-spectrum cephalosporins cefotaxime and ceftizoxime. The deduced amino acid sequence of SHV-2A shows that amino acid 238 is a serine, a residue reported to confer resistance to cefotaxime. We discuss how the operon may have evolved by a combination of insertion sequence-mediated genetic rearrangements and acquisitive evolution. Using phylogenetic parsimony, we show that aphA7 in the IAB operon evolved from an ancestral form similar to aphA1 in Tn903 and that blaS2A evolved from an ancestral form similar to blaS1.

Base Sequence↗

PubMed1990

Structural characterization of the Salmonella typhimurium LT2 umu operon.

The umuDC operon of Escherichia coli encodes functions required for mutagenesis induced by radiation and a wide variety of chemicals. The closely related organism Salmonella typhimurium is markedly less mutable than E. coli, but a umu homolog has recently been identified and cloned from the LT2 subline. In this study the nucleotide sequence and structure of the S. typhimurium LT2 umu operon have been determined and its gene products have been identified so that the molecular basis of umu activity might be understood more fully. S. typhimurium LT2 umu consists of a smaller 417-base-pair (bp) umuD gene ending 2 bp upstream of a larger 1,266-bp umuC gene. The only apparent structural difference between the two operons is the lack of gene overlap. An SOS box identical to that found in E. coli is present in the promoter region upstream of umuD. The calculated molecular masses of the umuD and umuC gene products were 15.3 and 47.8 kilodaltons, respectively, which agree with figures determined by transpositional disruption and maxicell analysis. The S. typhimurium and E. coli umuD sequences were 68% homologous and encoded products with 71% amino acid identity; the umuC sequences were 71% homologous and encoded products with 83% amino acid identity. Furthermore, the potential UmuD cleavage site and associated catalytic sites could be identified. Thus the very different mutagenic responses of S. typhimurium LT2 and E. coli cannot be accounted for by gross differences in operon structure or gene products. Rather, the ability of the cloned S. typhimurium umuD gene to give stronger complementation of E. coli umuD77 mutants in the absence of a functional umuC gene suggests that Salmonella UmuC protein normally constrains UmuD protein activity.

Amino Acid Sequence↗

PubMed1991

Cloning, nucleotide sequence, and expression of the Bacillus subtilis ans operon, which codes for L-asparaginase and L-aspartase.

L-Aspartase was purified from Bacillus subtilis, its N-terminal amino acid sequence was determined to construct a probe for the aspartase gene, and the gene (termed ansB) was cloned and sequenced. A second gene (termed ansA) was found upstream of the ansB gene and coded for L-asparaginase. These two genes were in an operon designated the ans operon, which is 80% cotransformed with the previously mapped aspH1 mutation at 215 degrees. Primer extension analysis of in vivo ans mRNA revealed two transcription start sites, depending on the growth medium. In wild-type cells in log-phase growth in 2x YT medium (tryptone-yeast extract rich medium), the ans transcript began at -67 relative to the translation start site, while cells in log-phase growth or sporulating (t1 to t4) in 2x SG medium (glucose nutrient broth-based moderately rich medium) had an ans transcript which began at -73. The level of the -67 transcript was greatly increased in an aspH mutant grown in 2x YT medium; the -67 transcript also predominated when this mutant was grown in 2x SG medium, although the -73 transcript was also present. In vitro transcription of the ans operon by RNA polymerase from log-phase cells grown in 2x YT medium and log-phase or sporulating cells grown in 2x SG medium yielded only the -67 transcript. Depending on the growth medium, the levels of asparaginase and aspartase were from 2- to 40-fold higher in an aspH mutant than in wild-type cells, and evidence was obtained indicating that the gene defined by the aspH1 mutation codes for a trans-acting transcriptional regulatory factor. In wild-type cells grown in 2x SG medium, the levels of both aspartase and asparaginase decreased significantly by t0 of sporulation but then showed a small increase, which was mirrored by changes in the level of beta-galactosidase from an ansB-lacZ fusion. The increase in the activities of ans operon enzymes between t2 and t5 of sporulation was found primarily in the forespore, and the great majority of the increased was found in the mature spore. However, throughout sporulation the only ans transcript detected was the -73 form, and no sporulation-specific RNA polymerase tested yielded a -73 transcript in vitro.

Amino Acid Sequence↗

PubMed1991

A base pair transition in a DNA sequence with dyad symmetry upstream of the puf promoter affects transcription of the puc operon in Rhodobacter capsulatus.

A DNA sequence with dyad symmetry upstream of the transcriptional start of the Rhodobacter capsulatus puf operon, which encodes pigment-binding proteins of the light-harvesting I complex and of the reaction center, has previously been shown to be a protein-binding site (G. Klug, Mol. Gen. Genet. 226:167-176, 1991). When a low-copy-number plasmid with a base pair transition at position -43 within this dyad symmetry in front of the puf structural genes was transferred into a Rhodobacter strain with the puf operon deleted, different phenotypes occurred during cultivation of the transconjugants and the kinetics of the loss of the wild-type phenotype was dependent on the oxygen tension in the culture. After growth for 150 generations, the different phenotypes were stably inherited. The strains having the wild-type phenotype carried the wild-type puf DNA sequence. The original mutation was still present in the strains that showed lighter color. These strains had less light-harvesting II complex in the membrane and showed lower rates of transcription of the puc operon, which encodes the proteins of this complex. This deregulation of puc expression was due to one or more chromosomally located, secondary mutations, not directly to the mutation present on the plasmid. Thus, a single-base-pair transition in the puf upstream region can result in a deregulation of puc expression, suggesting a direct or indirect transcriptional coregulation of both these operons by a common factor.

Bacterial Proteins↗

PubMed1991

Effect of regB on expression from the P1 and P2 promoters of the Pseudomonas aeruginosa regAB operon.

Exotoxin A production in Pseudomonas aeruginosa is dependent on two regulatory genes, regA and regB, which are located in tandem on the chromosome. Expression of regA and regB is controlled by two promoters (P1 and P2) situated upstream of the regAB locus. We have studied the effect of the regA and regB gene products on transcription from the regAB promoters. Transcriptional and translational fusions, under the control of the P. aeruginosa regA promoters, were used to analyze the regulation of these promoters in a variety of genetic backgrounds. When the regA P1 promoter was supplied in trans to strains lacking expression of regB (PAO1) or lacking transcription of the regAB operon (PA103-29), little activity from the P1 promoter was detected. In contrast, activity from the P2 promoter was not affected in either PAO1 or PA103-29. Sequence analysis of the regAB operon of PA103-29 detected two mutations. One of the mutations is predicted to result in a premature stop codon in the regA open reading frame. We complemented PA103-29 with a construction containing regA and an inactive regB or a construction containing both regA and regB to directly analyze the effect of regB on transcription of the regAB operon. When PA103-29 was complemented with regA but not regB, we could not detect any transcription from the P1 promoter. Complementation of PA103-29 with both regA and regB resulted in a high level of transcription from the P1 promoter and a corresponding early transcriptional activation of toxA. Our results indicated that induction of transcription from the P1 promoter requires the regB open reading frame and thus the regAB operon is autogenously regulated in P. aeruginosa.

ADP Ribose Transferases↗

PubMed1992

Structure and regulation of the Yersinia pestis yscBCDEF operon.

We have investigated the physical and genetic structure and regulation of the Yersinia pestis yscBCDEF region, previously called lcrC. DNA sequence analysis showed that this region is homologous to the corresponding part of the ysc locus of Yersinia enterocolitica and suggested that the yscBCDEF cistrons belong to a single operon on the low-calcium response virulence plasmid pCD1. Promoter activity measurements of ysc subclones indicated that yscBCDEF constitutes a suboperon of the larger ysc region by revealing promoter activity in a clone containing the 3' end of yscD, intact yscE and yscF, and part of yscG. These experiments also revealed an additional weak promoter upstream of yscD. Northern (RNA) analysis with a yscD probe showed that operon transcription is thermally induced and downregulated in the presence of Ca2+. Primer extension of operon transcripts suggested that two promoters, a moderate-level constitutive one and a stronger, calcium-downregulated one, control full-length operon transcription at 37 degrees C. Primer extension provided additional support for the proposed designation of a yscBCDEF suboperon by identifying a 5' end within yscF, for which relative abundances in the presence and absence of Ca2+ revealed regulation that is distinct from that for transcripts initiating farther upstream. YscB and YscC were expressed in Escherichia coli by using a high-level transcription system. Attempts to express YscD were only partially successful, but they revealed interesting regulation at the translational level.

Bacterial Proteins↗

PubMed1992

Localization of upstream sequence elements required for nitrate and anaerobic induction of fdn (formate dehydrogenase-N) operon expression in Escherichia coli K-12.

Two transcriptional activators, the FNR and NARL proteins, are required for induction of the fdnGHI operon, encoding Escherichia coli formate dehydrogenase-N. The FNR protein is required for anaerobic expression, while the NARL protein mediates nitrate induction. We used primer extension to locate the transcription initiation site 29 nucleotides upstream of the fdnG translation initiation codon. Expression assays with single-copy phi (fdnG-lacZ) gene fusions containing various deletions in the fdn 5'-regulatory region delimited three distinct cis-acting elements. One site, which is located at approximately -110, was required for nitrate induction. Two other sites share sequence similarity with the FNR protein binding site core consensus. The first site, centered at -42.5, was required for anaerobic induction. We used site-specific mutagenesis to change this putative FNR protein binding site into the CRP protein binding site core consensus. This change caused the fdn operon to be expressed aerobically, subject to CRP protein control. On the other hand, converting this putative FNR protein binding site into the FNR protein binding site core consensus resulted in elevated anaerobic induction of the fdn operon and also caused weak aerobic expression. The other putative FNR protein binding site, centered at -97.5, was not involved in anaerobic induction. It might play a negative role in fdn operon expression during anaerobic growth in the absence of nitrate.

Anaerobiosis↗

PubMed1992

Multiple repetitive elements and organization of the lux operons of luminescent terrestrial bacteria.

The complete nucleotide sequences of the luxA to luxE genes, as well as the flanking regions, were determined for the lux operons of two Xenorhabdus luminescens strains isolated from insects and humans. The nucleotide sequences of the corresponding lux genes (luxCDABE) were 85 to 90% identical but completely diverged 350 bp upstream of the first lux gene (luxC) and immediately downstream of the last lux gene (luxE). These results show that the luxG gene found immediately downstream of luxE in luminescent marine bacteria is missing at this location in terrestrial bacteria and raise the possibility that the lux operons are at different positions in the genomes of the X. luminescens strains. Four enteric repetitive intergenic consensus (ERIC) or intergenic repetitive unit (IRU) sequences of 126 bp were identified in the 7.7-kbp DNA fragment from the X.luminescens strain isolated from humans, providing the first example of multiple ERIC structures in the same operon including two ERIC structures at the same site. Only a single ERIC structure between luxB and luxE is present in the 7-kbp lux DNA from insects. Analysis of the genomic DNAs from five X. luminescens strains or isolates by polymerase chain reaction has demonstrated that an ERIC structure is between luxB and luxE in all of the strains, whereas only the strains isolated from humans had an ERIC structure between luxD and luxA. The results indicate that there has been insertion and/or deletion of multiple 126-bp repetitive elements in the lux operons of X.luminescens during evolution.

Amino Acid Sequence↗

PubMed1992

Nucleotide and deduced amino acid sequences of the lacR, lacABCD, and lacFE genes encoding the repressor, tagatose 6-phosphate gene cluster, and sugar-specific phosphotransferase system components of the lactose operon of Streptococcus mutans.

The complete nucleotide sequences of lacRABCDF and partial nucleotide sequence of lacE from the lactose operon of Streptococcus mutans are presented. Comparison of the streptococcal lac determinants with those of Staphylococcus aureus and Lactococcus lactis indicate exceptional protein and nucleotide identity. The deduced polypeptides also demonstrate significant, but lower, sequence similarity with the corresponding lactose proteins of Lactobacillus casei. Additionally, LacR has sequence homology with the repressor (DeoR) of the Escherichia coli deoxyribonucleotide operon, while LacC is similar to phosphokinases (FruK and PfkB) from E. coli. The primary translation products of the lacRABCDFE genes are polypeptides of 251 (M(r) 28,713), 142 (M(r) 15,610), 171 (M(r) 18,950), 310 (M(r) 33,368), 325 (M(r) 36,495), 104 (M(r) 11,401), and 123 (NH2-terminal) amino acids, respectively. As inferred from their direct homology to the staphylococcal lac genes, these determinants would encode the repressor of the streptococcal lactose operon (LacR), galactose-6-phosphate isomerase (LacA and LacB), tagatose-6-phosphate kinase (LacC), tagatose-1,6-bisphosphate aldolase (LacD), and the sugar-specific components enzyme III-lactose (LacF) and enzyme II-lactose (LacE) of the S. mutans phosphoenolpyruvate-dependent phosphotransferase system. The nucleotide sequence encompassing the S. mutans lac promoter appears to contain repeat elements analogous to those of S. aureus, suggesting that repression and catabolite repression of the lactose operons may be similar in these organisms.

Amino Acid Sequence↗

PubMed1992

Molecular analysis of the Escherichia coli phoP-phoQ operon.

The phoP-phoQ operon of Salmonella typhimurium is a member of the family of two-component regulatory systems and controls expression of the phoN gene that codes for nonspecific acid phosphatase and the genes involved in the pathogenicity of the bacterium. The phoP-phoQ operon of Escherichia coli was cloned on a plasmid vector by complementation of a phoP mutant, and the 4.1-kb nucleotide sequence, which includes the phoP-phoQ operon and its flanking regions, was determined. The phoP-phoQ operon was mapped at 25 min on the standard E. coli linkage map by hybridization with the Kohara mini set library of the E. coli chromosome (Y. Kohara, K. Akiyama, and K. Isono, Cell 50:495-508, 1987). The predicted phoP and phoQ gene products consist of 223 and 486 amino acids with estimated molecular masses of 25,534 and 55,297 Da, respectively, which correspond well with the sizes of the PhoP and PhoQ proteins identified by the maxicell method. The amino acid sequences of PhoP and PhoQ of E. coli were 93 and 86% identical, respectively, to those of S. typhimurium.

Amino Acid Sequence↗

PubMed1992

Identification of a B subunit gene promoter in the Shiga toxin operon of Shigella dysenteriae 1.

The Shiga toxin operon (stx) is composed of A and B subunit genes which are transcribed as a bicistronic mRNA from a promoter which lies 5' to the stxA gene. Northern (RNA) blot and primer extension analyses revealed the existence of a second stxB gene transcript. Recombinant plasmids which carried the stxB gene without the stx operon promoter and with the influence of a vector promoter abrogated produced STX B polypeptides, suggesting that the stxB gene mRNA was transcribed from an independent promoter and was not produced by endoribonucleotic processing of the bicistronic mRNA. Examination of the DNA sequences 5' to the stxB gene transcription initiation site which were carried by the recombinant plasmids revealed a region with high homology to the consensus for Escherichia coli promoters. Deletion and mutation of this region affected StxB and holotoxin production, establishing its role in the regulation of the stxB gene. Comparison of the promoters by using a transcription analysis vector revealed that the stxB gene promoter differed from the stx operon promoter in that was approximately sixfold less efficient and was not repressed by iron. Identification of a second promoter in the stx operon indicates that independent transcription of the stxB gene may regulate overproduction of the STX B polypeptides and may contribute to the 1A:5B subunit stoichiometry of the holotoxin.

Amino Acid Sequence↗

PubMed1992

Determination of the sequence of spaE and identification of a promoter in the subtilin (spa) operon in Bacillus subtilis.

An 851-residue open reading frame (ORF) called SpaE has been discovered in the subtilin (spa) operon. Interruption of this ORF with a chloramphenicol acetyltransferase gene destroys the ability of Bacillus subtilis LH45 delta c (a derivative of B. subtilis 168) to produce subtilin, which is an antimicrobial peptide belonging to the class of ribosomally synthesized peptide antibiotics called lantibiotics. SpaE shows strong homology to NisB, which is in the nisin (nis) operon in Lactococcus lactis ATCC 11454. Despite the strong sequence homology between SpaE and NisB, the spaE and nisB genes occupy very different locations in their respective operons, indicating that they have been evolving separately for a long time. Primer extension analysis was employed to identify a promoter upstream from the spaE gene, which appears to define the 5' end of the spa operon, which contains four other ORFs (Y. J. Chung, M. T. Steen, and J. N. Hansen, J. Bacteriol. 174:1417-1422, 1992).

Amino Acid Sequence↗

PubMed1992

The Calvin cycle enzyme pentose-5-phosphate 3-epimerase is encoded within the cfx operons of the chemoautotroph Alcaligenes eutrophus.

Several genes (cfx genes) encoding Calvin cycle enzymes in Alcaligenes eutrophus are organized in two highly homologous operons comprising at least 11 kb. One cfx operon is located on the chromosome; the other is located on megaplasmid pHG1 of the organism (B. Bowien, U. Windhövel, J.-G. Yoo, R. Bednarski, and B. Kusian, FEMS Microbiol. Rev. 87:445-450, 1990). Corresponding regions of about 2.7 kb from within the operons were sequenced. Three open reading frames, designated cfxX (954 bp), cfxY (765 bp), and cfxE (726 bp), were detected at equivalent positions in the two sequences. The nucleotide identity of the sequences amounted to 94%. Heterologous expression of the subcloned pHG1-encoded open reading frames in Escherichia coli suggested that they were functional genes. The observed sizes of the gene products CfxX (35 kDa), CfxY (27 kDa), and CfxE (25.5 kDa) closely corresponded to the values calculated on the basis of the sequence information. E. coli clones harboring the cfxE gene showed up to about 19-fold-higher activities of pentose-5-phosphate 3-epimerase (PPE; EC 5.1.3.1) than did reference clones, suggesting that cfxE encodes PPE, another Calvin cycle enzyme. These data agree with the finding that in A. eutrophus, PPE activity is significantly enhanced under autotrophic growth conditions which lead to a derepression of the cfx operons. No functions could be assigned to CfxX and CfxY.

Alcaligenes↗

PubMed1992

The lethal phenotype caused by null mutations in the Escherichia coli htrB gene is suppressed by mutations in the accBC operon, encoding two subunits of acetyl coenzyme A carboxylase.

Insertion mutations in the Escherichia coli htrB gene result in the unique phenotype of not affecting growth at temperatures below 32.5 degrees C but leading to a loss of viability at temperatures above this in rich media. When htrB bacteria growing in rich media were shifted to the nonpermissive temperature of 42 degrees C, they continued to grow at a rate similar to that at 30 degrees C but they produced phospholipids at the rate required for growth at 42 degrees C. This led to the accumulation of more than twice as much phospholipid per milligram of protein compared with that in wild-type bacteria. Consistent with HtrB playing a role in phospholipid biosynthesis, one complementation group of spontaneously arising mutations that suppressed htrB-induced lethality were mapped to the accBC operon. This operon codes for the biotin carboxyl carrier protein and biotin carboxylase subunits of the acetyl coenzyme A carboxylase enzyme complex, which catalyzes the first step in fatty acid biosynthesis. Four suppressor mutations mapped to this operon. Two alleles were identified as mutations in the accC gene, the third allele was identified as a mutation in the accB gene, and the fourth allele was shown to be an insertion of an IS1 transposable element in the promoter region of the operon, resulting in reduced transcription. The suppressor mutations caused a decrease in the rate of phospholipid biosynthesis, restoring the balance between the biosynthesis of phospholipids and growth rate, thus enabling htrB bacteria to grow at high temperatures.

Acetyl-CoA Carboxylase↗

PubMed1992

Genes encoding the phycobilisome rod substructure are clustered on the Anabaena chromosome: characterization of the phycoerythrocyanin operon.

The phycoerythrocyanin (pec) operon, cloned from Anabaena sp. strain PCC 7120, encodes four genes, pecBACE, located upstream of the C-phycocyanin (cpc) operon. This pec-cpc cluster includes all the genes for the structural components of the phycobilisome rod. Oligonucleotide probes based on the amino-terminal sequence of the phycoerythrocyanin beta subunit were used to clone an 8.0-kbp EcoRI fragment which was determined, by sequencing, to partially overlap the previously cloned cpc operon. A 5.0-kbp EcoRI-ClaI fragment corresponding to the region upstream of the cpc operon was subsequently subcloned and sequenced. Five open reading frames whose polarity of transcription is parallel to that of the cpc genes were identified. pecB and pecA encode the beta and alpha subunits of phycoerythrocyanin, respectively. pecC encodes the phycoerythrocyanin-associated linker polypeptide LR34.5,PEC. The identities of these genes are confirmed by agreement with amino-terminal sequences determined from purified phycobilisome components. A gene homologous to cpcE, found downstream of pecC, has been designated pecE. The cpcE gene product is involved in the attachment of the phycocyanobilin chromophore to the alpha subunit of phycocyanin. Three transcripts were observed by Northern (RNA) analyses. The most abundant of these transcripts, 1.35 kbp, corresponds to the beta and alpha subunit genes, whereas the less-abundant transcripts, 2.3 and 3.1 kbp, correspond to pecBAC and pecBACE, respectively. Phycoerythrocyanin is strongly induced in cells cultured under low light. In parallel, all three transcripts were present at much higher levels in cells cultured under low light.

Amino Acid Sequence↗

PubMed1993

Heat shock transcription of the groESL operon of Agrobacterium tumefaciens may involve a hairpin-loop structure.

The groESL operon of Agrobacterium tumefaciens was cloned and sequenced and found to be highly homologous to previously analyzed groE operons in nucleotides of the coding region and in amino acid sequence. Transcription of this operon in A. tumefaciens was considerably stimulated by heat shock. Primer extension analysis revealed that the groE transcripts from cells under heat shock were initiated from the same promoter (a sigma-70-like promoter) as transcripts from untreated cells, and no sequence homology with the Escherichia coli heat shock promoters was observed. The DNA sequence downstream of the transcription start site contains an inverted repeat that has a strong similarity to other groESL operons of both gram-positive and gram-negative bacteria (such as cyanobacteria and chlamydiae). This conserved region is thought to form a hairpin-loop structure and may play a role in gene regulation during heat shock.

Agrobacterium tumefaciens↗

PubMed1993

Inhibition of expression of the tryptophanase operon in Escherichia coli by extrachromosomal copies of the tna leader region.

Expression of the tryptophanase (tna) operon in Escherichia coli is regulated by catabolite repression and transcription attenuation. Expression is induced by the presence of elevated levels of tryptophan in a growth medium devoid of a catabolite-repressing carbon source. Induction requires the translation of a 24-residue coding region, tnaC, located in the 319-nucleotide transcribed leader region preceding tnaA, the structural gene for tryptophanase. Multicopy plasmids carrying the tnaC leader region were found to inhibit induction of the chromosomal tna operon. Mutational studies established that this inhibition was not due to inhibited transcription initiation, translation initiation, tryptophan transport, or enzyme activity. Rather, multicopy tnaC plasmids inhibited induction by preventing tryptophan-induced transcription antitermination in the leader region of the tna operon. Translation of the single Trp codon in tnaC of the multicopy plasmids was shown to be essential for this inhibition. We hypothesize that translation of the Trp codon of the leader peptide titrates out a trans-acting factor that is essential for tryptophan-induced antitermination in the chromosomal tna operon. We postulate that this factor is an altered form of tRNATrp.

Alleles↗