ANALYSIS OF A HET(-) MUTATION IN ANABAENA SP STRAIN PCC-7120 IMPLICATES A SECONDARY METABOLITE IN THE REGULATION OF HETEROCYST SPACING

Transposon-generated mutant N10 of Anabaena sp. strain PCC 7120 has a Het(-) phenotype (A. Ernst, T. Black, Y. Cai, J.-M. Panoff, D. N. Tiwa ri, and C. P. Wolk, J. Bacteriol. 174:6025-6032, 1992). Reconstruction of the transposon mutation reproduced a Het(-) phenotype, but reconst ructions with other insertions at the position of the transposon produ ced strains that form multiple contiguous heterocysts. Sequence analys is around the site of insertion of the transposon showed that the inse rtion lies within the 5' end of an 861-bp open reading frame (ORF) (he tN). The product of translation of hetN (HetN) shows extensive similar ity to NAD(P)H-dependent oxidoreductases that are involved in biosynth eses of fatty acids, poly-beta-hydroxybutyrate, nod factor, and polyke tides. A second, 1,518-bp ORF (hetM) that ends 556 bp 5' from the star t of hetN appears to encode a protein that has at least two functional domains: its amino terminus is similar to an acyl carrier protein, wh ile its central portion is similar to domains of proteins that perform reductive reactions. A third, 711-bp ORF (hetI) encoded on the opposi te strand ends 42 bp away from the 3' end of hetN. The protein encoded by hetI, HetI, is similar to Sfp from Bacillus subtilis and EntD from Escherichia coli, proteins that are required for the biosynthesis or export of cyclic peptides. Clones from a lambda-EMBL3 library that con tain the wild-type DNA for hetN do not complement the hetN::Tn5-1063 m utation in N10. The presence of hetN, as the only ORF, on a replicatin g plasmid suppresses heterocyst formation in wild-type cells, whereas the additional presence of hetI alleviates this effect.