Suppression of heterocyst differentiation in Anabaena PCC 7120 by a cosmid carrying wild-type genes encoding enzymes for fatty acid synthesis.

A cosmid containing a wild-type Anabaena PCC 7120 DNA fragment was found to suppress heterocyst differentiation, creating a Het phenotype in an otherwise wild-type strain. Curing of the cosmid restored the full wild-type Het+ Nif+ phenotype. The cosmid contains at least four genes encoding proteins with significant sequence similarity to enzymes involved in the synthesis of fatty acids. Selection for Nif+ revertants of the suppressed strain yielded modified cosmids, one of which contained a 10.2-kb transposon, Tas1, inserted into the promoter region of a gene encoding a protein with acyl carrier and beta-keto reductase domains. This gene, called hetN, was shown previously by Black and Wolk (J. Bacteriol. (1994) 176, 2282-2292) to inhibit heterocyst differentiation when present alone on a plasmid. Oddly, hetN gene transcription is detected later than 6 h into heterocyst differentiation.