SECG AND TEMPERATURE MODULATE EXPRESSION OF AZIDE-RESISTANT AND SIGNAL SEQUENCE SUPPRESSOR PHENOTYPES OF ESCHERICHIA-COLI SECA MUTANTS

SecA is a dynamic protein that undergoes ATP-dependent membrane cyclin g to drive protein translocation across the Escherichia coli inner mem brane. To understand more about this process, azide-resistant (azi) an d signal sequence suppressor (prlD) alleles of secA were studied. We f ound that azide resistance is cold sensitive because of a direct effec t on protein export., suggesting that SecA-membrane interaction is reg ulated by an endothermic step that is azide inhibitable. secG function is required for expression of azide-resistant and signal sequence sup pressor activities of azi and prlD alleles, and in turn, these alleles suppress cold-sensitive and export-defective phenotypes of a secG nul l mutant. These remarkable genetic observations support biochemical da ta indicating that SecG promotes SecA membrane cycling and that this p rocess is dependent on an endothermic change in SecA conformation.