REGULATION OF HEPA OF ANABAENA SP. STRAIN PCC-7120 BY ELEMENTS 5' FROM THE GENE AND BY HEPK

In Anabaena spp., synthesis of the heterocyst envelope polysaccharide, required if the cell is to fix dinitrogen under aerobic conditions, i s dependent on the gene hepA. A transcriptional start site of hepA was localized 104 bp 5' from its translational initiation codon. A 765-bp open reading frame, denoted hepC, was found farther upstream. Inactiv ation of hepC led to constitutive expression of hepA and prevented the synthesis of heterocyst envelope polysaccharide. However, the glycoli pid layer of the heterocyst envelope was synthesized. A hepK mutation blocked both the synthesis of the heterocyst envelope polysaccharide a nd induction of hepA. The predicted product of hepK resembles a sensor y protein-histidine kinase of a two-component regulatory system. Analy sis of the region between hepC and hepA indicated that DNA sequences r equired for the induction of hepA upon nitrogen deprivation are presen t between bp -574 and -440 and between bp -340 and -169 relative to th e transcriptional start site of hepA, Gel mobility shift assays provid ed evidence that one or more proteins bind specifically to the latter sequence. The Fox box sequence downstream from hepA appeared inessenti al for the induction of hepA.