| Description |
The Escherichia coli htrB gene was identified during an insertional mutagenesis screen for new heat shock genes. HtrB is essential for viability in rich media only at temperatures above 32.5°C, a phenotype related to bacterial growth rate, since htrB bacteria are viable at high temperatures in minimal medium. Despite its unique temperature-sensitive phenotype, the htrB gene is not under heat shock regulation. When grown at nonpermissive temperatures, htrB bacteria exhibit density-dependent morphological alterations, including the formation of bulges and filaments. The lipopolysaccharide layer of the outer membrane may be altered in htrB bacteria, as indicated by the ability of cationic detergents to reverse the lethal phenotype and by the increased resistance of htrB bacteria to deoxycholate. Four spontaneously arising mutations that suppress the HtrB temperature-sensitive phenotype were mapped to the accBC operon, encoding two of the subunits of acetyl-CoA carboxylase, which catalyzes the first step in fatty acid biosynthesis. Biochemical analysis indicates that htrB mutant bacteria overproduce phospholipids at nonpermissive temperatures, a phenotype closely correlated with loss in viability. The accBC mutations most likely suppress the lethal phenotype of htrB by lowering the rate of fatty acid biosynthesis, thus inhibiting the phospholipid overproduction. Two new genes were also identified in this study as multicopy suppressors of htrB. The protein encoded by the msbA suppressor is related to the ATP-dependent translocator family of proteins involved in the export of molecules out of cells. The msbA gene is a unique member of this family because it is essential for bacterial viability. The orfE gene, which is coexpressed with msbA, is also essential. The protein encoded by the msbB suppressor gene appears to play a similar, if not redundant role to HtrB, because MsbB and HtrB have similar amino acid sequences and structural feature. Furthermore, htrB msbB double mutant bacteria exhibit both morphological alterations and growth defects at 30°C, phenotypes that are not exhibited by either of the single mutants. Although bacteria with msbB null mutations are viable, they also exhibit an increased resistance to deoxycholate, indicating that like HtrB, MsbB may play a role in outer membrane function. |
