Immunolocalization of NblA, a protein involved in phycobilisome turnover, during heterocyst differentiation in cyanobacteria

Organismes Photosynthétiques et Environnement, CNRS FRE 2433, Département de Biologie, École Normale Supérieure, 46 rue d'Ulm, 75230 Paris Cedex 05, France Correspondence Jean Houmard jhoumard{at}biologie.ens.fr In unicellular non-diazotrophic cyanobacteria, NblA is a small polypeptide required for phycobilisome degradation during macronutrient limitation. In the filamentous N 2 -fixing Tolypothrix sp., a nblA gene ( nblAI ) lies upstream of the cpeBA operon that encodes phycoerythrin apoproteins. Using a specific anti-NblAI antibody it was found that in strains of Tolypothrix sp. NblAI abundance increases under nitrogen-limiting conditions but the protein is also present in cells grown in nitrogen-replete medium. Gold immunolabelling experiments showed that, upon a nitrogen shift-down, NblAI is preferentially located in the differentiated heterocysts, where O 2 evolution has to be shut off for nitrogenase to operate. The results lead to the proposal that NblAI is a necessary ‘cofactor’ but not the triggering factor that governs phycobilisome degradation in Tolypothrix sp. Present address: Facultad de C.C. Biológicas, Universidad S.E.K., C/ Cardenal Zúñiga s/n, 40003 Segovia, Spain.