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S R Wessler

Publications and source records attributed to S R Wessler.

54 records · Page 3Linked to original sources
PubMed1991

The maize transposable Ds1 element is alternatively spliced from exon sequences.

The null wx-ml allele contains a 409-bp Dissociation 1 (Ds1) element in exon 9 of the maize waxy (Wx) gene. In the absence of the autonomous Activator (Ac) element, the Ds1 element cannot transpose, and this allele encodes several Wx transcripts that arise following alternative splicing of Ds1 sequences from Wx pre-mRNA. Splicing involves the utilization of three 5' splice sites and three 3' splice sites. All but one of these splice sites are in Ds1 sequences near the ends of the element. The presence of 5' and 3' splice sites near the Ds1 termini and the element's small size and AT richness are features that distinguish Ds1 elements from all other known Ds elements. It is suggested that these features may enhance the ability of Ds1 to function as a mobile intron.

Alleles↗
PubMed1990

Implications for the cis-requirements for Ds transposition based on the sequence of the wxB4 Ds element.

The nucleotide sequence of the 1494 bp wxB4 Ds element is presented. A comparison with previously characterized Ds elements reveals several novel features. This element has less Ac terminal sequence than other Ac-like Ds elements. The left terminus contains 398 bp of Ac sequence interrupted by a transposon-like DNA insertion, leaving only 317 bp of contiguous Ac sequence. The right terminus has 259 bp of Ac terminal sequence. The interior of the element contains sequences not found in other cloned members of the Ac/Ds family. We suggest that the role of this non-Ac DNA is to separate the Ac termini by a minimum distance and may be a cis requirement for Ds transposition in maize.

Base Sequence↗
PubMed1989

The splicing of maize transposable elements from pre-mRNA--a minireview.

There are six examples of maize transposable elements that are spliced from pre-mRNA. These represent the first introns that have been added to nuclear genes in recent years. All six are members of the Ac/Ds or Spm/dSpm family of elements and all have been inserted into exons of active genes within the last twenty-five years. The structure of these element-introns and the sequences involved in their splicing are presented. These examples illustrate how active transposable elements can also function as introns and how they may evolve into stable introns.

Alleles↗
PubMed1989

Lc, a member of the maize R gene family responsible for tissue-specific anthocyanin production, encodes a protein similar to transcriptional activators and contains the myc-homology region.

Previous studies have suggested that the R locus of maize is responsible for determining the temporal and spatial pattern of anthocyanin pigmentation in the plant. In this report we demonstrate that three members of the R gene family, P, S, and Lc, encode homologous transcripts 2.5 kilobases in length. The structure of one R gene, Lc, was determined by sequencing cDNA and genomic clones. The putative Lc protein, deduced from the cDNA sequence, is composed of 610 amino acids and has homology to the helix-loop-helix DNA-binding/dimerization motif found in the L-myc gene product and other regulatory proteins. It also contains a large acidic domain that may be involved in transcriptional activation. Consistent with its proposed role as a transcriptional activator is our finding that a functional R gene is required for the accumulation of transcripts of at least two genes in the anthocyanin biosynthetic pathway. We discuss the possibility that the diverse patterns of anthocyanin pigmentation conditioned by different R genes reflect differences in the R gene promoters rather than their gene products.

Amino Acid Sequence↗
PubMed1988

Phenotypic diversity mediated by the maize transposable elements Ac and Spm.

Mutations caused by the insertion of members of the Ac or Spm family of transposable elements result in a great diversity of phenotypes. With the cloning of the mutant genes and the characterization of their products, the mechanisms underlying phenotypic diversity are being deciphered. These mechanisms include (i) imprecise excision of transposable elements, which can result in the addition of amino acids to proteins; (ii) DNA methylation, which has been correlated with the activity of the element; (iii) transposase-mediated deletions within elements, which can inactivate an element or lead to a new unstable phenotype; and (iv) removal of transcribed elements from RNA, which can facilitate gene expression despite the insertion of elements into exons. An understanding of the behavior of the maize elements has provided clues to the function of cryptic elements in all maize genomes.

Alleles↗
PubMed1988

An RFLP adjacent to the maize waxy gene has the structure of a transposable element.

Two maize inbred lines harbor non-mutant waxy (Wx) genes that display restriction fragment length polymorphism (RFLP) upstream from the start of Wx transcription. Sequencing of this region in the two strains revealed a DNA insertion with the structural features of a transposable element. The insertion is 316 bp in length, has 15 bp imperfect inverted repeats and is flanked by a 5 bp direct repeat generated upon insertion. Sequences homologous to this insertion are present in multiple copies in maize and its relatives teosinte and Tripsacum but not in the more distantly related dicot tobacco. Finally, this element is not homologous with any previously described maize DNA insertion.

Base Sequence↗
PubMed1988

Comparison of non-mutant and mutant waxy genes in rice and maize.

The waxy gene, which is responsible for the synthesis of amylose in endosperm and pollen, is genetically well characterized in many grasses including maize and rice. Homology between the previously cloned maize waxy gene and the rice gene has facilitated our cloning of a 15-kb HindIII fragment that contains the entire rice gene. A comparison of the restriction maps of the maize and rice genes indicates that many restriction sites within translated exons are conserved. In addition, the rice gene encodes a 2.4-kb transcript that programs the in vitro synthesis of a 64-kD pre-protein which is efficiently precipitated with maize waxy antisera. We demonstrate that these gene products are altered in rice strains containing mutant waxy genes. Southern blot analysis of 16 rice strains, ten containing waxy mutations, reveals that the waxy gene and flanking restriction fragments are virtually identical. These results contrast dramatically with the high level of insertions and deletions associated with restriction fragment length polymorphism and spontaneous mutations among the waxy alleles of maize.

Alleles↗
PubMed1987

The maize transposable element Ds is spliced from RNA.

In some instances, insertion of maize transposable elements into exons does not result in the total loss of enzymatic activity. In other instances, messenger RNAs of wild-type size are encoded by genes known to contain the maize transposable element Dissociation (Ds) in exons. To understand how Ds is processed from RNA, a study was made of transcripts encoded by two alleles of the maize waxy (wx) gene containing Ds insertions in exon sequences. The analysis was carried out in strains where the Ds element could not excise from the wx gene. Despite insertions of 4.3- and 1.5-Ds elements, the predominant transcripts encoded by these two genes were wild type in size. For both alleles, DNA sequencing of complementary DNAs revealed that the Ds elements had been spliced in a similar manner. Splicing was accomplished by the utilization of multiple 5' donor splice sites in the Ds termini and a 3' acceptor site within the wx gene adjacent to the Ds element. The net effect in both cases was the removal of most of the Ds element from the messenger RNA.

Base Sequence↗
PubMed1986

Excision of Ds produces waxy proteins with a range of enzymatic activities.

The waxy (wx) locus of maize encodes an enzyme responsible for the synthesis of amylose in endosperm tissue. The phenotype of the Dissociation (Ds) insertion mutant wx-m1 is characterized by endosperm sectors that contain different levels of amylose. We have cloned the Wx gene from this allele and from two germinal derivatives, S5 and S9, that produce intermediate levels of amylose. The Ds insertion in wx-m1 is in exon sequences, is 409 bp in length and represents an example of a class of Ds elements that are not deletion derivatives of the Activator (Ac) controlling element. The two germinal derivatives, S5 and S9, lack the Ds element but contain an additional 9 and 6 bp, respectively, at the site of Ds insertion. The level of Wx mRNA and Wx protein in S5 and S9 is essentially the same as in normal endosperm tissue but Wx enzymatic activity is reduced. Thus, the lesions in S5 and S9 lead to the addition of amino acids in the Wx protein, resulting in Wx enzymes with altered specific activities. This work supports the notion that the maize transposable elements may serve a function in natural populations to generate genetic diversity, in this case, proteins with new enzymatic properties.

Alleles↗
PubMed1986

High A + T content conserved in DNA sequences upstream of leuABCD in Escherichia coli and Salmonella typhimurium.

The nucleotide sequence of over 800 base pairs of DNA upstream of leuP was determined for Escherichia coli and Salmonella typhimurium. In both of these enteric bacteria, approximately 500 base pairs of A + T-rich sequences separates leuP from an upstream open reading frame. Although these A + T-rich sequences share little homology, the distribution of A + T base pairs within the region is strikingly conserved. Deletion of the A + T-rich sequences upstream of the E. coli leu operon does not markedly affect the strength of the leu promoter in vivo.

Adenine↗
PubMed1985

Molecular basis of mutations at the waxy locus of maize: correlation with the fine structure genetic map.

More than 40 mutant alleles of the waxy (Wx) locus of maize are available for molecular analysis. Previous studies have examined the nature of phenotypically unstable Wx mutant alleles caused by insertion of the maize transposable activator (Ac) and dissociation (Ds) elements. In this study we have used Southern blot analysis to characterize the locus in 22 strains harboring wx alleles with stable mutant phenotypes. Of these mutations, 17 are of spontaneous origin, 4 were induced by gamma rays, and 1 was induced by ethyl methanesulfonate. Of these 22 alleles, we find that 13 have either insertions or deletions within the Wx transcription unit. The insertions range in size from 150 base pairs to 6.1 kilobases. For 4 of the 6 deletions identified, the two breakpoints are within the Wx gene. For 9 other alleles we can detect no obvious lesions within or around the transcription unit. Evidence is presented that the insertions and deletions result in the mutant phenotype and are not polymorphisms. This conclusion is based on two findings: (i) a survey of inbred lines revealed only a single instance of polymorphism within the transcription unit, whereas all of the lesions described alter the transcription unit; and (ii) there is an excellent correlation between the position of these lesions on the physical map and their relative position on a fine structure genetic map of the locus.

Alleles↗
PubMed1985

Functional determinants in transit sequences: import and partial maturation by vascular plant chloroplasts of the ribulose-1,5-bisphosphate carboxylase small subunit of Chlamydomonas.

The precursor of the ribulose-1,5-bisphosphate carboxylase small subunit and other proteins from Chlamydomonas reinhardtii are efficiently transported into chloroplasts isolated from spinach and pea. Thus, similar determinants specify precursor-chloroplast interactions in the alga and vascular plants. Removal of all or part of its transit sequence was found to block import of the algal small subunit into isolated chloroplasts. Comparison of available sequences revealed a nine amino acid segment conserved in the transit sequences of all small subunit precursors. A protease in the vascular plant chloroplasts recognized this region in the Chlamydomonas precursor and produced an intermediate form of the small subunit. We propose that processing of the small subunit precursor involves at least two proteolytic events; only one of these has been evolutionarily conserved.

Amino Acid Sequence↗
PubMed1983

Transcription attenuation is the major mechanism by which the leu operon of Salmonella typhimurium is controlled.

Three mutations, each causing constitutive expression of the Salmonella typhimurium leu operon, were cloned into phage vector lambda gt4 on EcoRI DNA fragments carrying all of that operon except for part of the promoter-distal last gene. Sequence analysis of DNA from these phage demonstrated that each contains a single base change in the leu attenuator. Transcription of mutant DNA in vitro resulted in transcription beyond the usual site of termination. The level of beta-IPM dehydrogenase, the leuB enzyme, was elevated 40-fold in a strain carrying one of these mutations, and starvation of this strain for leucine had little effect on the amount of activity expressed. Using a strain with a wild-type promoter-leader region of the leu operon, the rates of synthesis and degradation of leu leader RNA and readthrough RNA (leu mRNA) were measured by DNA-RNA hybridizations with specific DNA probes. The rate of synthesis of the leu leader was about the same in cells grown with excess or with limiting leucine. On the other hand, the rate of synthesis of leu mRNA was 12-fold higher for cells grown in limiting leucine as opposed to excess leucine. The rate of degradation of these RNA species was the same under both conditions of growth. Thus, the variation in expression of the leu operon observed for cells grown in minimal medium is, for the most part, not caused by control over the frequency of initiation or by the differential stability of these RNA species. Rather, the variation is a direct result of the frequency of transcription termination at an attenuator site. These results taken together suggest that transcription attenuation is the major mechanism by which leucine regulates expression of the leu operon of S. typhimurium for cells growing in a minimal medium.

Bacteriophage lambda↗
PubMed1981

Physical characterization of the ilvHI operon of Escherichia coli K-12.

The ilvHI and leu genes of Escherichia coli K-12 are contained on a single 10.9-kilobase EcoRI fragment of deoxyribonucleic acid derived from the leu transducing phage lambda G4. Since the expression of all of these genes is controlled by leucine, we investigated whether they are part of single operon or whether they constitute separate but adjacent operons controlled from a common site. Both cloning and hybridization studies indicated that ilvHI and leu are distinct operons. They are transcribed in opposite directions and are separated by approximately 1,500 base pairs of deoxyribonucleic acid. Hybridization experiments showed that the expression of ilvHI is regulated chiefly at the level of transcription. The size of the ilvHI messenger ribonucleic acid is estimated to be 2,550 bases.

Acetolactate Synthase↗
PubMed1979

leu operon of Salmonella typhimurium is controlled by an attenuation mechanism.

The nucleotide sequence of the control region of the leu operon of Salmonella typhimurium was determined. A prominent feature of this region is a signal for termination of transcription. In vitro, transcription does terminate at this site, yielding a leader RNA of about 160 nucleotides as a major product. This leader RNA is potentially translatable into a peptide containing 28 amino acids, 4 of which are adjacent leucine residues. Several regions of base complementarity exist within the leader, positioned such that pairing of one region precludes pairing of another. The position of the four leucine codons relative to two regions of base complementarity suggest a model for the regulation of the leu operon similar to that proposed by Yanofsky and coworkers for the trp operon. In addition, a third region of base complementarity was identified which, when incorporated into the model, explains why premature termination is the usual outcome when transcription is initiated in vitro by purified RNA polymerase.

Base Sequence↗