CLONING AND CHARACTERIZATION OF THE GENES-CODING FOR 2 PORINS IN THE UNICELLULAR CYANOBACTERIUM SYNECHOCOCCUS PCC-6301

The genes somB and somA (Synechococcus outer membrane), lying in tande m organization in the genome of Synechococcus PCC 6301, encode two por ins 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 Som A and SomB protein sequences include typical N-terminal signal peptide s and reveal 60% homology (50% identical residues) to each other as we ll as significant homology to six protein sequences deduced from open reading frames sequenced in the genome of the unicellular cyanobacteri um Synechocystis PCC 6803. Furthermore, SomA possesses an overall iden tity of 97% to the functionally not yet characterized outer-membrane p rotein SomA from the closely related cyanobacterial strain Synechococc us PCC 7942. Analyses performed on the sequences suggest that SomA and SomB form 14- or 16-stranded porin-like beta-barrels. Moreover, all s equences share an N-terminal motif with significant homology to 'S-lay er homology' domains, which might form a periplasmic extension. SomA a nd SomB therefore may, in addition to their porin function, act as lin kers connecting the outer membrane with the peptidoglycan layer. (C) 1 998 Elsevier Science B.V. All rights reserved.