THE DEVBCA EXPORTER IS ESSENTIAL FOR ENVELOPE FORMATION IN HETEROCYSTS OF THE CYANOBACTERIUM ANABAENA SP. STRAIN PCC-7120

The gene devA of the filamentous heterocyst-forming cyanobacterium Ana baena sp. strain PCC 7120 encodes a protein with high similarity to AT P-binding cassettes of ABC transporters. Mutant M7 defective in the de vA gene is arrested in the development of heterocysts at an early stag e and is not able to fix N-2 under aerobic conditions. The devA gene i s differentially expressed in heterocysts. To gain a better understand ing of the structural components of this putative ABC transporter, we determined the complete nucleotide sequence of the entire gene cluster . The two additional genes, named devB and devC, encode proteins with similarities to membrane fusion proteins (DevB) of several ABC exporte rs and to membrane-spanning proteins (DevC) of ABC transporters in gen eral. Site-directed mutations in each of the three genes resulted in i dentical phenotypes. Heterocyst-specific glycolipids forming the lamin ated layer of the envelope were identified in lipid extracts of M7 and in the site-directed mutants. However, transmission electron microsco py revealed unequivocally that the glycolipid layer is missing in muta nt M7. Ultrastructural analysis also confirmed a developmental block a t an early stage of differentiation. The results of this study suggest that the devBCA operon encodes an exporter of glycolipids or of an en zyme that is necessary for the formation of the laminated layer. The h ypothesis is proposed that an intact envelope could be required for fu rther heterocyst differentiation.