The outer membrane of a cyanobacterium ( sp. strain PCC 7942) contains only a few major proteins. A gene encoding one of them, was cloned and characterized. Based on the nucleotide sequence, SomA was predicted to comprise 531 amino acids with a calculated molecular mass of 57136 Da. The deduced amino acid sequence of SomA shares similarities with two bacterial cell-surface proteins, the S-layer protein of and the flagellin of 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 , provided that this sequence was fused to the 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.


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