Biomedical subjects

D I Andersson

Publications and source records attributed to D I Andersson.

26 records · Page 2Linked to original sources

PubMed1992

Cobalamin (vitamin B12) repression of the Cob operon in Salmonella typhimurium requires sequences within the leader and the first translated open reading frame.

Expression of the Cob operon in Salmonella typhimurium is repressed by cobalamin (Cbl). Here it is shown that Cbl repression is mediated by a post-transcriptional regulatory mechanism that requires sequences within the leader and the first translated open reading frame, the cbiA gene. Transcriptional and translational Cob::lacZ fusions containing various lengths of Cob DNA were analysed. In a translational Cob::lacZ fusion 407 bp of leader sequence (+69 to +476) was sufficient to confer normal repression. However in a transcriptional Cob::lacZ fusion a 618 bp region (+69 to +687) was required for normal repression. This 618 bp region included sequences in the leader as well as sequences within the cbiA gene. Point mutations which resulted in loss of repression control were isolated and shown to be clustered in the leader sequence (+257 to +380). This region contains a putative hairpin-loop structure which we propose functions as an RNA operator site for a vitamin B12-responsive repressor protein.

Base Sequence↗

PubMed1991

Analysis of an anaerobically induced promoter for the cobalamin biosynthetic genes in Salmonella typhimurium.

We have identified an anaerobically induced promoter for the cobalamin biosynthetic (cob) genes. In a plasmid the Cob promoter showed two of the three types of control of the intact chromosomal Cob operon: anaerobic induction and cAMP stimulation. Cobalamin repression was observed only in promoter fragments which included sequences far downstream of the transcription start site, suggesting that this control is post-transcriptional. One anaerobically induced transcript was identified and its 5' end was determined. Deletion mapping showed that 60 nucleotides upstream of the start site were sufficient for anaerobic synthesis of this transcript. Upstream of the transcription start site a putative sigma 70-dependent -10 recognition sequence was identified; however, no consensus -35 region was observed.

Anaerobiosis↗

PubMed1989

Mutations affecting regulation of cobinamide biosynthesis in Salmonella typhimurium.

Transcription of the genes for cobalamin biosynthesis is reduced during aerobic growth. We isolated and characterized mutants that showed a 2- to 90-fold increase in aerobic expression of the cobinamide biosynthesis (CobI) genes, depending on the particular mutation and growth conditions. Four different classes of mutations were characterized. All mutations (CobRI through CobRIV) were cis-acting, dominant mutations that mapped near the promoter end of the CobI operon. Two of these classes of mutations (III and IV) caused an increase in anaerobic as well as aerobic transcription of the CobII and CobIII operons; this led to increased biosynthesis of cobalamin under anaerobic growth conditions. A recessive mutation (cobF) mapping far from the CobI operon increased anaerobic CobI operon expression by about fourfold.

Chromosome Mapping↗

PubMed1989

Redox regulation of the genes for cobinamide biosynthesis in Salmonella typhimurium.

Transcription of the cobinamide biosynthetic genes (the CobI operon) was induced under three different physiological conditions: anaerobiosis (anaerobic respiration or fermentation), aerobic respiration at low oxygen levels, and aerobic respiration with a partial block of the electron transport chain. After a shift to inducing conditions, there was a time lag of approximately 50 min before the onset of CobI induction. Under conditions of anaerobic respiration, the level of CobI transcription was dependent on the nature of both the electron donor (carbon and energy source) and the acceptor. Cells grown with electron acceptors with a lower midpoint potential showed higher CobI expression levels. The highest level of CobI transcription observed was obtained with glycerol as the carbon source and fumarate as the electron acceptor. The high induction seen with glycerol was reduced by mutational blocks in the glycerol catabolic pathway, suggesting that glycerol does not serve as a gratuitous inducer but must be metabolized to stimulate CobI transcription. In the presence of oxygen, CobI operon expression was induced 6- to 20-fold by the following: inhibition of cytochrome o oxidase with cyanide, mutational blockage of ubiquinone biosynthesis, and starvation of mutant cells for heme. We suggest that the CobI operon is induced in response to a reducing environment within the cell and not by the absence of oxygen per se.

Aerobiosis↗

PubMed1986

Suboptimal growth with hyper-accurate ribosomes.

Mutant bacteria with hyperaccurate ribosomes support their excessive accuracy of translation in vitro by dissipating 1.5 to 2.5 cognate ternary complexes per peptide bond formed. This is to be compared with a dissipation rate close to 1.1 for wild-type ribosomes. Here, we have tested the hypothesis that a corresponding loss of translational efficiency in vivo would lower the growth rate of the mutants. Such a growth inhibitory effect would explain why the lower accuracy of wild-type ribosomes is more fit. Our data show that as expected the mu of the hyperaccurate mutants is smaller than that of wild-type bacteria. In contrast, during glucose-limited growth in chemostats there is not the same simple correlation between growth yield and ribosomal efficiency for the hyperaccurate mutants.

Bacteria↗

PubMed1986

Functional interactions between mutated forms of ribosomal proteins S4, S5 and S12.

Here we show that ram mutations, either in ribosomal protein S4 or S5, decrease the proofreading flows for both cognate and noncognate ternary complexes bound by streptomycin-dependent (SmD) ribosomes. This effect is accompanied by a slight increase in the overall error frequency. More important, however, is the decreased proofreading of the cognate species which is almost reduced to wild-type levels. The data suggest that it may be the reduction of the proofreading of the cognate substrate that is important for suppressing streptomycin dependence. Furthermore, we show that rpsE mutants, selected from streptomycin-dependent strains, behave kinetically very similarly to the previously described rpsD mutants.

Codon↗

PubMed1983

Ram ribosomes are defective proofreaders.

We have studied the kinetics of poly(U) translation by three ribosomal ambiguity (Ram) mutants in an in vitro system with performance characteristics similar to those expressed in vivo. The leucine missense frequency supported by Ram ribosomes with tRNALeu2 increases between six and twelve-fold over that of wild-type ribosomes, while the corresponding increase with tRNALeu4 was between four and eight-fold, depending on the rpsD allele. We have used a steady-state assay for proofreading to identify the kinetic lesion responsible for the Ram phenotype. We were unable to detect any difference between Ram and wild-type ribosomes with respect to the initial kinetics of amino-acyl tRNA selection. All of the increased error rates could be associated with a decreased capacity of these Ram ribosomes to discard non-cognate aminoacyl-tRNA by proof reading.

Escherichia coli↗

PubMed1982

Translation rates and misreading characteristics of rpsD mutants in Escherichia coli.

Three ribosomal ambiguity (Ram) mutants, changed in ribosomal protein S4, have been examined with respect to elongation rate and misreading of translation in vivo and in vitro. Ram mutants increase misreading of nonsense codons in vivo, compared to wild type, between 2-50 times depending on the nature of the nonsense codon, its position, and which rpsD allele is present. Ram ribosomes also show an increased error frequency in vitro. The elongation rate of translation does not seem to be significantly changed, neither in vivo nor in vitro, irrespective of which rpsD allele is present. We suggest that there exists no general relationship between the accuracy and the overall speed of translation in Ram strains.

Alleles↗