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 fou nd 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 enzyme s involved in the synthesis of fatty acids. Selection for Nif(+) rever tants of the suppressed strain yielded modified cosmids, one of which contained a 10.2-kb transposon, Tas1, inserted into the promoter regio n of a gene encoding a protein with acyl carrier and beta-keto reducta se domains. This gene, called hetN, was shown previously by Black and Wolk (J. Bacteriol. (1994) 176, 2282-2292) to inhibit heterocyst diffe rentiation when present alone on a plasmid. Oddly, hetN gene transcrip tion is detected later than 6 h into heterocyst differentiation.