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The genes somB and somA (Synechococcus outer membrane), lying in tandem organization in the genome of Synechococcus PCC 6301, encode two porins in the outer membrane of this unicellular cyanobacterium. Northern blot and primer extension experiments revealed that somA and somB are not comprising an operon, as each gene encodes a transcript of 1.7 kb length and has a distinct transcriptional start site. The deduced SomA and SomB protein sequences include typical N-terminal signal peptides and reveal 60% homology (50% identical residues) to each other as well as significant homology to six protein sequences deduced from open reading frames sequenced in the genome of the unicellular cyanobacterium Synechocystis PCC 6803. Furthermore, SomA possesses an overall identity of 97% to the functionally not yet characterized outer-membrane protein SomA from the closely related cyanobacterial strain Synechococcus PCC 7942. Analyses performed on the sequences suggest that SomA and SomB form 14- or 16-stranded porin-like beta-barrels. Moreover, all sequences share an N-terminal motif with significant homology to 'S-layer homology' domains, which might form a periplasmic extension. SomA and SomB therefore may, in addition to their porin function, act as linkers connecting the outer membrane with the peptidoglycan layer.

Two major proteins, A and B, were isolated and purified from outer membranes of the unicellular cyanobacterium Synechococcus PCC 6301 by gel filtration, anion-exchange chromatography, and preparative SDS-PAGE. Protein A revealed a single-channel conductance of 0.4 nanoSiemens (nS) in 1 M KCl, whereas preparations containing both proteins showed two different conductance maxima of 0.4 and 0.9 nS, suggesting that B also forms pores. The apparent molecular mass of the two closely migrating proteins was determined as 52 kDa, whereas native porin extracts revealed a relative molecular mass of ca. 140 kDa, indicating trimeric pore-forming units. Partial sequences of both proteins were obtained by N-terminal sequencing of tryptic peptides, and the C-terminal amino acid sequences were derived from the complete proteins. These sequences were aligned to protein sequences available in the databases. The results are discussed.

Pore-forming protein (porin) was isolated from N,N-dimethyl-dodecylaminoxid (LDAO)-extracted outer membranes of Synechococcus PCC 6301 and purified by ion exchange chromatography on DEAE-Sephacel column. The apparent molecular mass on SDS-PAGE was determined to be about 52,000. The native porin was reconstituted into black lipid bilayer membranes and showed a single-channel conductance of 5.5 nS in 1 M KCl. The porin was found to be N-terminally blocked. The C-terminal amino acid sequence was identified as Phe-Thr-Phe. Amino acid analysis suggested that the porin protein consists of about 420 amino acid residues, yielding a polarity of 43.6% and a molecular mass of 45,000 in contrast to the mobility on SDS-PAGE.