pbpB, a gene coding for a putative penicillin-binding protein, is requiredfor aerobic nitrogen fixation in the cyanobacterium Anabaena sp strain PCC7120

Transposon mutagenesis of Anabaena sp, strain PCC7120 led to the isolation of a mutant strain, SNa1, which is unable to fix nitrogen aerobically but i s perfectly able to grow with combined nitrogen (i.e., nitrate). Reconstruc tion of the transposon mutation of SNa1 in the wild-type strain reproduced the phenotype of the original mutant. The transposon had inserted within an open reading frame whose translation product shows significant homology wi th a family of proteins known as high-molecular-weight penicillin-binding p roteins (PBPs), which are involved in the synthesis of the peptidoglycan la yer of the cell wall. A sequence similarity search allowed us to identify a t least 12 putative PBPs in the recently sequenced Anabaena sp, strain PCC7 120 genome, which we have named and organized according to predicted molecu lar size and the Escherichia coli nomenclature for PBPs; based on this nome nclature, we have denoted the gene interrupted in SNal as pbpB and its prod uct as PBP2, The wild-type form of pbpB on a shuttle vector successfully co mplemented the mutation in SNa1, In vivo expression studies indicated that PBP2 is probably present when both sources of nitrogen, nitrate and N-2, ar e used, When nitrate is used, the function of PBP2 either is dispensable or may be substituted by other PBPs; however, under nitrogen deprivation, whe re the differentiation of the heterocyst takes place, the role of PBP2 in t he formation and/or maintenance of the peptidoglycan layer is essential.