Genetic characterization of Salmonella typhimurium LT2 ara mutations

Virulence Gene Regulation by L-Arabinose in Salmonella enterica

Invasion of the intestinal epithelium is a critical step in Salmonella enterica infection and requires functions encoded in the gene cluster known as Salmonella Pathogenicity Island 1 (SPI-1). Expression of SPI-1 genes is repressed by L-arabinose, and not by other pentoses. Transport of L-arabinose is necessary to repress SPI-1; however, repression is independent of L-arabinose metabolism and of the L-arabinose-responsive regulator AraC. SPI-1 repression by L-arabinose is exerted at a single target, HilD, and the mechanism appears to be post-translational. As a consequence of SPI-1 repression, l-arabinose reduces translocation of SPI-1 effectors to epithelial cells and decreases Salmonella invasion in vitro. These observations reveal a hitherto unknown role of L-arabinose in gene expression control and raise the possibility that Salmonella may use L-arabinose as an environmental signal.

Studies on the domain structure of the Salmonella typhimurium AraC protein

The Salmonella typhimurium araC gene product is known to be susceptible to proteolytic degradation. Limited cleavage by trypsin, kallikrein, elastase and pronase E yields stable fragments comprising approximately the N-terminal two thirds of the AraC protein. These fragments have in common the ability to dimerize in solution and to bind L-arabinose and D-fucose. Under appropriate conditions, hydrolysis of the AraC protein with Staphylococcus aureus V8 protease leads to a small C-terminal fragment which is able to bind specifically to a synthetic ara consensus sequence. These results indicate that, as with several other prokaryotic gene regulatory proteins, the basic functions of effector binding, subunit interaction and specific DNA binding are segregated into distinct domains of the AraC protein.

Role of gene duplications in the adaptation of Salmonella typhimurium to growth on limiting carbon sources

Duplication-containing cells are selected when growth of Salmonella typhimurium is limited by the availability of any one of several carbon and energy sources. Under conditions of extreme starvation, growth occurs almost exclusively in the duplication-containing fraction of the population. Cells with duplications of one large segment of the chromosome are repeatedly selected regardless of which of these carbon sources limits growth. The duplicated chromosomal segment encodes the transport systems for all of these carbon sources. This duplication is not selected during growth on a carbon source for which the permease is not included within the duplication segment. This suggests that the growth advantage conferred by the duplication may be due to increased transport of the limiting carbon source. Inclusion of the permease alone is not sufficient to explain the growth advantage of the duplications, since other common duplications that include the permease are not selected.

Cloning, genetic characterization, and nucleotide sequence of the hemA-prfA operon of Salmonella typhimurium

The first step in heme biosynthesis is the formation of 5-aminolevulinic acid (ALA). Mutations in two genes, hemA and hemL, result in auxotrophy for ALA in Salmonella typhimurium, but the roles played by these genes and the mechanism of ALA synthesis are not understood. I have cloned and sequenced the S. typhimurium hemA gene. The predicted polypeptide sequence for the HemA protein shows no similarity to known ALA synthases, and no ALA synthase activity was detected in extracts prepared from strains carrying the cloned hemA gene. Genetic analysis, DNA sequencing of amber mutations, and maxicell studies proved that the open reading frame identified in the DNA sequence encodes HemA. Another surprising finding of this study is that hemA lies directly upstream of prfA, which encodes peptide chain release factor 1 (RF-1). A hemA::Kan insertion mutation, constructed in vitro, was transferred to the chromosome and used to show that these two genes form an operon. The hemA gene ends with an amber codon, recognized by RF-1. I suggest a model for autogenous control of prfA expression by translation reinitiation.

Mutations resulting in promoter-like sequences which enhance the expression of araC in Salmonella typhimurium

The araC gene in Salmonella typhimurium is autogeneously regulated. Nine non-self-regulated mutants were isolated by selecting for increased expression of an araC-lacZ fusion in the presence of a repressing AraC protein. S1 mapping experiments demonstrated that the effect of the mutations was to increase the amount of araC mRNA in the cell. The 5'-end of the major araC transcript was the same in the wild-type (wt) and mutant strains. DNA sequence analysis showed that all nine mutations occurred in the araC promoter. Two mutations were a G-to-T transversion at position -25, six were a G-to-T transversion at position -47 and one was a single bp deletion at position -83. The data suggest that the mutations have created a new RNA polymerase binding site which enhances transcription from the wt start point.

The araBAD operon of Salmonella typhimurium LT2. I. Nucleotide sequence of araB and primary structure of its product, ribulokinase

Hybrid plasmids containing the araBAD operon of Salmonella typhimurium LT2 were characterized by Southern blot and genetic analyses. The nucleotide sequence of araB was determined. The araB gene product, ribulokinase (EC 2.7.1.16), was purified and the results of amino acid composition analysis and partial amino acid sequence are in agreement with predictions from the DNA sequence. Ribulokinase is 569 amino acid residues long and has a calculated Mr of 61 793. Ribulokinase shares significant homology with xylulose kinase from Escherichia coli. Codon usage in the araB gene does not favor those codons which have intermediate codon-anticodon binding energy.