Three strategies for genetic analysis show that two inner membrane components of the export machinery, PrlA (SecY) and PrlG (SecE), interact directly while catalyzing the translocation of secreted proteins across the cytoplasmic membrane of E. coli. The first, suppressor-directed inactivation (SDI), exploits the specific interaction between dominant prl suppressors of signal sequence mutations and mutant LacZ hybrid proteins. The second, Sec titration, extends SDI to allow the identification of various Sec proteins that are present in the translocation complex. The third uses the synthetic lethality of certain double-mutant strains to infer physical interactions between gene products. Biochemical data obtained with SDI strains allow the identification of two different secretory intermediates and indicate that PrlG functions before PrlA in the secretion pathway.