Cloning of a secA homolog from Streptomyces lividans 1326 and overexpression in both S-lividans and Escherichia coli

We cloned a gene encoding a SecA homolog from Streptomyces lividans 1326, a Gram-positive bacterium known to produce large amounts of extracellular proteins. A protein sequence alignment with the other bacterial SecA homologs revealed that S. lividans SecA shares from 39.5 to 44% identity with them, while it shares 34.2 to 37.2% identity with SecA homologs from plastids of algae and plants. We overexpressed the secA gene in S. lividans 1326 and Escherichia coli MM52 and in both cases we observed the production of a protein with an apparent molecular mass of 117.4 kDa. Although S. lividans SecA is similar to E. coli SecA, it does not complement a thermosensitive mutation in the E, coli secA gene. However, a hybrid polypeptide consisting of the N-terminal portion (first 242 amino acids) of the S. lividans SecA and the C-terminal portion (657 a.a.) of the wild-type E. coli SecA was able to complement this mutant.