A serine protease (SSP) of Serratia marcescens is one of the extracellular enzymes secreted from this Gram-negative bacterium. SSP is produced as a large precursor and converted to a mature protein by cleavages removing an NH₂-terminal signal sequence and a COOH-terminal pro-region. This COOH-terminal pro-region is integrated into the outer membrane and has a functional role for the export of the mature protein across the outer membrane. Southern hybridization analysis with a DNA fragment encoding the COOH-terminal pro-region as the probe showed a wide distribution of nucleotide sequences encoding SSP exporter-like proteins among Serratia species. Moreover, S. marcescens IFO 3046, from which the ssp gene had been cloned, was found to contain two ssp homologues (ssp-h1 and ssp-h2). They were cloned and their nucleotide sequences were determined. The two ssp homologues were found to exist in tandem on the genome and their amino acid sequences showed 81% identity to each other. Both of them showed 55% identity in amino acid sequence to preproSSP. In addition, both showed end-to-end similarity to the 100 kDa serotype-specific antigen (Ssa1) of Pasteurella haemolytica. Escherichia coli JM105 containing ssp-h1 gene produced a 53 kDa protein corresponding to the NH₂terminal portion and a 49 kDa protein corresponding to the COOH-terminal portion, both of which were rigidly integrated in the outer membrane. Consistent with the significant similarity of the COOH-terminal portions of the homologues to that of SSP, they showed the ability to translocate the mature SSP part across the outer membrane into the medium. Furthermore, the NH₂-terminal portion of the homologue was not translocated into the outer membrane without its COOH-terminal part. All of these data show that the SSP homologues are outer membrane proteins that are translocated into the outer membrane with the aid of the translocator function of their COOH-terminal part.