G. Fiedler et al., THE DEVBCA EXPORTER IS ESSENTIAL FOR ENVELOPE FORMATION IN HETEROCYSTS OF THE CYANOBACTERIUM ANABAENA SP. STRAIN PCC-7120, Molecular microbiology, 27(6), 1998, pp. 1193-1202
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.