EXPRESSION OF GPSA ENCODING BIOSYNTHETIC SN-GLYCEROL 3-PHOSPHATE DEHYDROGENASE SUPPRESSES BOTH THE LB- PHENOTYPE OF A SECB NULL MUTANT AND THE COLD-SENSITIVE PHENOTYPE OF A SECG NULL MUTANT

SecB maintains the structures of a subset of precursor proteins compet ent for translocation across the Escherichia coli cytoplasmic membrane . SecG, a membrane component of the translocation machinery, stimulate s protein translocation by undergoing the cycle of membrane topology i nversion. Null mutants of secB and secG are unable to form isolated co lonies on rich medium and at low temperature respectively. A 3.2 kb DN A fragment carrying the secB-gpsA region on a multicopy plasmid was fo und to suppress the null mutation of either gene. However, subcloning of the DNA fragment revealed that secB is not involved in the suppress ion of either mutation. Instead, gpsA located downstream from the secB gene was found to be responsible for the suppression of both mutation s. The activity of the gpsA-encoded sn-glycerol-3-phosphate dehydrogen ase, which is involved in phospholipid synthesis, was significantly lo wer in the secB null mutant than in the wild type, presumably because of a polar effect. Suppression of the secB null mutation required the wild-type level of GpsA activity. In contrast, overexpression of the e nzyme was essential for suppression of the secG null mutation. Moreove r, the gpsA-dependent suppression of the secG null mutation occurred o nly on rich medium, i.e. not on minimal medium. These results indicate that the SecB function is dispensable even in rich medium, and furthe r demonstrate that overexpression of enzymes involved in phospholipid synthesis partly compensates for the SecG function.