A 3RD GENETIC FOCUS REQUIRED FOR THE FORMATION OF HETEROCYSTS IN ANABAENA SP STRAIN PCC-7120

Mutagenesis of Anabaena sp. strain PCC 7120 with a derivative of trans poson Tn5 led to the isolation of a mutant strain, P6, in which hetero cysts are not formed (A. Ernst, T. Black, Y. Cai, J.-M. Panoff, D. N. Tiwari, and C. P. Wolk, J. Bacteriol. 174:6025-6032, 1992). Reconstruc tion of the transposon mutation of P6 in the wild-type strain reproduc ed the phenotype of the original mutant. Analysis by pulsed-field gel electrophoresis localized the transposition at ca. 3.44 Mb on the phys ical map of the chromosome of wild-type Anabaena sp. The transposon wa s situated within an open reading frame (ORF), which we denote hetP, w hose wild-type form was cloned and also sequenced. The predicted HetP protein was not found to show significant sequence similarity to other proteins. The mutation in strain P6 could be complemented by a clone of a fragment of wild-type DNA that includes hetP and at least one add itional ORF 3' from hetP, but not by a clone that includes hetP as its only ORF. The latter clone proved highly toxic. The phenotype of the P6 mutant may, therefore, be due to a polar effect of the insertion of the transposon. Filaments of strain P6 and of the wild-type strain, w hen bearing the complementing fragment on a pDU1-based plasmid, showed an increased frequency of clustered heterocysts compared with that of the wild-type strain.