Mutations at three genetic loci (termed prlA,B,C) were previously shown to specifically suppress signal sequence mutations in the lamB gene encoding the outer membrane phage lambda receptor protein of Escherichia coli (Emr, S. D., Hanley-Way, S., and Silhavy, T. J. (1981) Cell 23, 79-88). The majority of these suppressor mutations map at the prlA locus and are thought to result in an altered ribosomal protein. In this study, we demonstrate that prlA mutations also phenotypically suppress signal sequence mutations in the malE gene encoding the periplasmic maltose-binding protein. For both lamB and malE mutations, suppression is achieved by transporting the export-defective protein to its correct extracytoplasmic location, in some instances with near 100% efficiency. With a single exception, the mutant-exported protein is apparently processed to its normal mature form. These results indicate that prlA-mediated protein export occurs via the usual route, and additional data suggest that the prlA product directly interacts with the mutant signal sequence to restore export. The single prlC allele also suppresses malE signal sequence mutations, whereas the single prlB allele only phenotypically suppresses lamB signal sequence mutations. However, with these latter two suppressors, there is some indication that export of the phage lambda receptor to the outer membrane is not accomplished by the usual route.