The control region of the pdu/cob regulon in Salmonella typhimurium.

The pdu operon encodes proteins for the catabolism of 1,2-propanediol; the nearby cob operon encodes enzymes for the biosynthesis of adenosyl-cobalamin (vitamin B12), a cofactor required for the use of propanediol. These operons are transcribed divergently from distinct promoters separated by several kilobases. The regulation of the two operons is tightly integrated in that both require the positive activator protein PocR and both are subject to global control by the Crp and ArcA proteins. We have determined the DNA nucleotide sequences of the promoter-proximal portion of the pdu operon and the region between the pdu and cob operons. Four open reading frames have been identified, pduB, pduA, pduF, and pocR. The pduA and pduB genes are the first two genes of the pdu operon (transcribed clockwise). The pduA gene encodes a hydrophobic protein with 56% amino acid identity to a 10.9-kDa protein which serves as a component of the carboxysomes of several photosynthetic bacteria. The pduF gene encodes a hydrophobic protein with a strong similarity to the GlpF protein of Escherichia coli, which facilitates the diffusion of glycerol. The N-terminal end of the PduF protein includes a motif for a membrane lipoprotein-lipid attachment site as well as a motif characteristic of the MIP (major intrinsic protein) family of transmembrane channel proteins. We presume that the PduF protein facilitates the diffusion of propanediol. The pocR gene encodes the positive regulatory protein of the cob and pdu operons and shares the helix-turn-helix DNA binding motif of the AraC family of regulatory proteins. The mutations cobR4 and cobR58 cause constitutive, pocR-independent expression of the cob operon under both aerobic and anaerobic conditions. Evidence that each mutation is a deletion creating a new promoter near the normal promoter site of the cob operon is presented.     

Over-synthesis and instability of sigma protein in a merodiploid strain of Escherichia coli.

Summary We have used two different methods to study the rates of RNA polymerase subunit synthesis in haploid Escherichia coli K12, and a KLF10 rpoB,C ⁺ merodiploid derivative, when grown in glucose-minimal medium at 37°C. Our results indicate that the haploid strain produces , , and in the molar ratios, 1.01:0.99:2.90:0.26; and that all these subunits are reasonably stable during subsequent growth. The merodiploid produces at the same rate as the haploid, and at a 42% higher rate, and sigma at twice the rate. Some 40% of the newly synthesised and is degraded within one hour; the residuum is as stable as in the haploid. is stable throughout. By contrast, sigma is subject to a marked and continuous turnover in the merodiploid. These results are discussed in terms of gene dosage and regulatory effects.     

Phosphate starvation regulon of Salmonella typhimurium.

Several phosphate-starvation-inducible (psi) genetic loci in Salmonella typhimurium were identified by fusing the lacZ gene to psi promoters by using the Mu d1 and Mu d1-8 bacteriophages. Although several different starvation conditions were examined, the psi loci responded solely to phosphate deprivation. A regulatory locus, psiR, was identified as controlling the psiC locus. The psiR locus did not affect the expression of the Escherichia coli phoA locus or any of the other psi loci described.     

Characterization of the fliL gene in the flagellar regulon of Escherichia coli and Salmonella typhimurium.

filL is a small gene of unknown function that lies within the beginning of a large flagellar operon of Salmonella typhimurium and Escherichia coli. A spontaneous fliL mutant of S. typhimurium, containing a frameshift mutation about 40% from the 3' end of the gene, was moderately motile but swarmed poorly, suggesting that FliL might be a component of the flagellar motor or switch. However, in-frame deletions of the E. coli gene, including an essentially total deletion, had little or no effect on motility or chemotaxis. Thus, FliL does not appear to have a major role in flagellar structure or function and is therefore unlikely to be a component of the motor or switch; the effect on motility caused by truncation of the gene is probably an indirect one.     

Contrasting mechanisms of envZ control of mal and pho regulon genes in Escherichia coli.

The envZ11 missense mutation in the regulatory gene envZ pleiotropically repressed synthesis of OmpF, alkaline phosphatase, and several proteins of the maltose regulon. Procaine treatment of wild-type cells resulted in the same phenotype through an envZ+-mediated mechanism. Here we show that envZ11-procaine act differently on the mal and pho regulons. In the mal system, the expression of the positive regulator gene malT, measured as beta-galactosidase activity of a malT-lac+ operon fusion, was drastically reduced by procaine treatment or by the envZ11 mutation. In contrast, expression of the positive regulator of the pho regulon phoB was not reduced by procaine treatment. The products of the regulatory genes phoM, phoR, and phoU were also not required for procaine action. Procaine and envZ11 inhibited expression of only two products of the pho regulon, alkaline phosphatase and the PhoE porin. The conclusion that envZ11-procaine act differently on the mal and the pho regulons is supported by our ability to isolate second-site mutations with a Mal+ PhoA- phenotype in an envZ11 strain.     

Osmoregulation of the maltose regulon in Escherichia coli.

The maltose regulon consists of four operons that direct the synthesis of proteins required for the transport and metabolism of maltose and maltodextrins. Expression of the mal genes is induced by maltose and maltodextrins and is dependent on a specific positive regulator, the MalT protein, as well as on the cyclic AMP-catabolite gene activator protein complex. In the absence of an exogenous inducer, expression of the mal regulon was greatly reduced when the osmolarity of the growth medium was high; maltose-induced expression was not affected, and malTc-dependent expression was only weakly affected. Mutants lacking MalK, a cytoplasmic membrane protein required for maltose transport, expressed the remaining mal genes at a high level, presumably because an internal inducer of the mal system accumulated; this expression was also strongly repressed at high osmolarity. The repression of mal regulon expression at high osmolarity was not caused by reduced expression of the malT, envZ, or crp gene or by changes in cellular cyclic AMP levels. In strains carrying mutations in genes encoding amylomaltase (malQ), maltodextrin phosphorylase (malP), amylase (malS), or glycogen (glg), malK mutations still led to elevated expression at low osmolarity. The repression at high osmolarity no longer occurred in malQ mutants, however, provided that glycogen was present.     

Synthesis of ribosomal proteins in merodiploid strains of Escherichia coli

Summary The regulation of the synthesis of r-proteins in Escherichia coli was investigated by increasing the dosage of the genes for a limited number of ribosomal proteins (r-proteins) using either transducing phage fus 3 (Lindahl et al. 1977) or rif ^d18 (Kirschbaum and Konrad 1973). During exponential growth the presence in the cell of either lysogenised transducing phage did not increase the rate of synthesis or degradation of any of the 31 r-proteins whose genes are duplicated. Experiments were also performed to determine whether r-protein synthesis during the period of unbalanced r-protein synthesis that follows nutritional enrichment was sensitive to an increase in gene dosage. Duplication of the 27 r-protein genes on fus 3 did not alter the rate of synthesis of any of the r-proteins after enrichment. However, gene dosage effects were detected for at least 3 of the r-proteins whose genes were duplicated of rif ^d18.     

Positive control of a regulon for defenses against oxidative stress and some heat-shock proteins in Salmonella typhimurium

S. typhimurium become resistant to killing by hydrogen peroxide and other oxidants when pretreated with nonlethal levels of hydrogen peroxide. During adaptation to hydrogen peroxide, 30 proteins are induced. Nine are constitutively overexpressed in dominant hydrogen peroxide-resistant oxyR mutants. Mutant oxyR1 is resistant to a variety of oxidizing agents and overexpresses at least five enzyme activities involved in defenses against oxidative damage. Deletions of oxyR are recessive and uninducible by hydrogen peroxide for the nine proteins overexpressed in oxyR1, demonstrating that oxyR is a positive regulatory element. The oxyR1 mutant is also more resistant than the wild-type parent to killing by heat, and it constitutively overexpresses three heat-shock proteins. The oxyR regulatory network is a previously uncharacterized global regulatory system in enteric bacteria.     

Salmonella SsrB activates a global regulon of horizontally acquired genes

Salmonella enterica is a bacterial pathogen of humans that can proliferate within epithelial cells as well as professional phagocytes of the immune system. This ability requires an S. enterica specific locus termed Salmonella pathogenicity island 2 (SPI-2). SPI-2 encodes a type III secretion system that injects effectors encoded within the island into host cell cytosol to promote virulence. SsrAB is a two-component regulator encoded within SPI-2 that was assumed to activate SPI-2 genes exclusively. Here, it is shown that SsrB in fact activates a global regulon. At least 10 genes outside SPI-2 are SsrB regulated within epithelial and macrophage cells. Nine of these 10 SsrB-regulated genes outside SPI-2 reside within previously undescribed regions of the Salmonella genome. Most share no sequence homology with current database entries. However, one is remarkably homologous to human glucosyl ceramidase, an enzyme involved in the ceramide signalling pathway. The SsrB regulon is modulated by the two-component regulatory systems PhoP/PhoQ and OmpR/EnvZ, and is upregulated in the intracellular microenvironment.