somA, a novel gene that encodes a major outer-membrane protein of Synechococcus sp. PCC 7942

The outer membrane of a cyanobacterium (Synechococcus sp. strain PCC 7942) contains only a few major proteins. A gene encoding one of them, somA, was cloned and characterized. Based on the nucleotide sequence, SomA was predicted to comprise 531 amino acids with a calculated molecular mass of 57,136 Da. The deduced amino acid sequence of SomA shares similarities with two bacterial cell-surface proteins, the S-layer protein of Thermus thermophilus and the flagellin of Campylobacter coli. The predicted amino acid sequence of SomA revealed also that it contains a signal peptide-like sequence at its N terminus. This signal peptide-like sequence was capable of mediating protein translocation across the cytoplasmic membrane into the outer membrane of Escherichia coli, provided that this sequence was fused to the E. coli outer-membrane protein, OmpF. The signal peptide-like sequence was cleaved upon the translocation of the SomA::OmpF protein. We suggest that SomA is synthesized as a precursor and that its N-terminal 24 amino acid sequence is a cleavable signal peptide involved in protein targeting into the outer membrane. To our knowledge, this is the first example of cleavable signal peptides for proteins transported into the outer membrane of cyanobacteria.