Reversion and immobilization of phage Mud1 cts (Apr lac) insertion mutations in Salmonella typhimurium

Mutations that render the promoter of the histidine operon of Salmonella typhimurium insensitive to nutrient-rich medium repression and amino acid downshift

The effects of mutations in the promoter of the histidine operon of Salmonella typhimurium were examined in vivo. The wild-type chromosomal copy of the his promoter was replaced with mutations in the -10 hexamer sequence and in the region between the -10 hexamer and the transcriptional start point-termed the discriminator sequence. The substitutions were performed with a phage M13 allele replacement system. Expression of the his operon is known to correlate with levels of guanosine 5',3'-bispyrophosphate (ppGpp) in vivo. Strains containing either the wild-type his promoter or his promoter mutations were grown in both nutrient-rich and minimal media under steady-state conditions known to alter intracellular levels of ppGpp in a predictable way. The effect of the presence or absence of the his attenuator was assessed under these conditions as well. Expression of the his operon was studied by measuring the differential rate of beta-galactosidase synthesis with a his-lac transcriptional fusion. Regulation of the his operon in the promoter mutants was also studied under conditions of a transient amino acid downshift induced by the addition of serine hydroxamate to cultures growing in nutrient-rich medium. These growth conditions cause elevated levels of ppGpp. The results provide physiological confirmation of previous evidence obtained with a coupled transcription-translation system in vitro which indicated that ppGpp regulates interaction of RNA polymerase at the his promoter. More specifically, the in vivo evidence shows that the region of the his promoter that includes the -10 hexamer and discriminator sequences is the target at which ppGpp stimulates transcription.

Mutational analysis of the histidine operon promoter of Salmonella typhimurium

We isolated a collection of 67 independent, spontaneous Salmonella typhimurium his operon promoter mutants with decreased his expression. The mutants were isolated by selecting for resistance to the toxic lactose analog o-nitrophenyl-beta-D-thiogalactoside in a his-lac fusion strain. The collection included base pair substitutions. small insertions, a deletion, and one large insertion identified as IS30 (IS121), which is resident on the Mu d1 cts(Apr lac) phage used to construct the his-lac fusion. Of the 37 mutations that were sequenced, 14 were unique. Six of the 14 were isolated more than once, with the IS30 insertion occurring 16 times. The mutations were located throughout the his promoter region, with two in the conserved - 35 hexamer sequence, four in the conserved - 10 hexamer sequence (Pribnow box), seven in the spacer between the - 10 and -35 hexamer sequences, and the IS30 insertions just upstream of the -35 hexamer sequence. Four of the five substitution mutations changed a consensus base pair recognized by E sigma 70 RNA polymerase in the -10 or -35 hexamer. Decreased his expression caused by the 14 different his promoter mutations was measured in vivo. Relative to the wild-type promoter, the mutations resulted in as little as a 4-fold decrease to as much as a 357-fold decrease in his expression, with the largest decreases resulting from changes in the most highly conserved features of E sigma 70 promoters.

Correlation between histidine operon expression and guanosine 5'-diphosphate-3'-diphosphate levels during amino acid downshift in stringent and relaxed strains of Salmonella typhimurium

We have analyzed the correlation of attenuator-independent expression of the Salmonella typhimurium histidine operon in vivo with levels of the "alarmone" guanosine 5'-diphosphate 3'-diphosphate. Amino acid downshift caused by serine hydroxamate addition increased his expression in a relA+ strain and decreased his expression in a relA mutant, whereas levels of guanosine 5'-diphosphate-3'-diphosphate varied in parallel with the changes in his expression in the two strains. In several experiments, overall variations in his expression ranged from 20- to 60-fold after downshift. The mild downshift allowed growth of the cultures to continue at near-preshift rates. Serine hydroxamate addition was also used to analyze the effect of amino acid downshift on induced expression of wild-type and mutant lac promoters. There was a 12-fold difference in lac expression when a relA+-relA1 pair was subjected to mild starvation but only a 3-fold difference when the strains carried the lacZpL8UV5 promoter mutation. These results suggest that guanosine 5'-diphosphate-3'-diphosphate stimulates gene expression in vivo at the level of transcription initiation.

Reduced leu operon expression in a miaA mutant of Salmonella typhimurium

Salmonella typhimurium miaA mutants lacking the tRNA base modification cis-2-methylthioribosylzeatin (ms2io6A) were examined and found to be sensitive to a variety of chemical oxidants and unable to grow aerobically at 42 degrees C in a defined medium. Leucine supplementation suppressed both of these phenotypes, suggesting that leucine synthesis was defective. Intracellular levels of leucine decreased 40-fold in mutant strains after a shift from 30 to 42 degrees C during growth, and expression of a leu-lacZ transcriptional fusion ceased. Steady-state levels of leu mRNA were also significantly reduced during growth at elevated temperatures. Failure of miaA mutant leu-lacZ expression to be fully derepressed during L-leucine limitation at 30 degrees C and suppression of the miaA mutation by a mutation in the S. typhimurium leu attenuator suggests that translational control of the transcription termination mechanism regulating leu expression is defective. Since the S. typhimurium miaA mutation was also suppressed by the Escherichia coli leu operon in trans, phenotypic differences between E. coli and S. typhimurium miaA mutants may result from a difference between their respective leu operons.

Escherichia coli genes whose products are involved in selenium metabolism

Mutants of Escherichia coli were isolated which were affected in the formation of both formate dehydrogenase N (phenazine methosulfate reducing) (FDHN) and formate dehydrogenase H (benzylviologen reducing) (FDHH). They were analyzed, together with previously characterized pleiotropic fdh mutants (fdhA, fdhB, and fdhC), for their ability to incorporate selenium into the selenopolypeptide subunits of FDHN and FDHH. Eight of the isolated strains, along with the fdhA and fdhC mutants, maintained the ability to selenylate tRNA, but were unable to insert selenocysteine into the two selenopolypeptides. The fdhB mutant tested had lost the ability to incorporate selenium into both protein and tRNA. fdhF, which is the gene coding for the 80-kilodalton selenopolypeptide of FDHH, was expressed from the T7 promoter-polymerase system in the pleiotropic fdh mutants. A truncated polypeptide of 15 kilodaltons was formed; but no full-length (80-kilodalton) gene product was detected, indicating that translation terminates at the UGA codon directing the insertion of selenocysteine. A mutant fdhF gene in which the UGA was changed to UCA expressed the 80-kilodalton gene product exclusively. This strongly supports the notion that the pleiotropic fdh mutants analyzed possess a lesion in the gene(s) encoding the biosynthesis or the incorporation of selenocysteine. The gene complementing the defect in one of the isolated mutants was cloned from a cosmid library. Subclones were tested for complementation of other pleiotropic fdh mutants. The results revealed that the mutations in the eight isolates fell into two complementation groups, one of them containing the fdhA mutation. fdhB, fdhC, and two of the new fdh isolates do not belong to these complementation groups. A new nomenclature (sel) is proposed for pleiotropic fdh mutations affecting selenium metabolism. Four genes have been identified so far: selA and selB (at the fdhA locus), selC (previously fdhC), and selD (previously fdhB).