Biosynthesis of a plasmid-encoded outer membrane surface exclusion protein involves processing from a precursor polypeptide.

MRB is a major R222 plasmid-encoded outer membrane protein previously described by this laboratory which is immunologically identical with F plasmid traTp. We have detected an apparent precursor polypeptide of MRB in Escherichia coli minicells which reacts with specific anti-MRB serum. In the presence of proteolytic inhibitors, immunologically reactive precursor MRB polypeptides are found in whole cells and minicells containing R222 or F plasmids. When R222-containing minicells were incubated for short periods of time with [35S]methionine, the precursor was predominantly labeled but was shown to be converted into MRB during subsequent incubation in media containing nonradioactive methionine. This conversion was inhibited in the presence of the proteolytic inhibitor tosyllysylalanyl chloramethyl ketone. These studies show that at least one plasmid-encoded membrane protein is processed in its E. coli host in a way resembling that of host cell membrane proteins.