Mutations in Four Regulatory Genes Have Interrelated Effects on Heterocyst Maturation in Anabaena sp. Strain PCC 7120

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Abstract
Regulatory genes hepK, hepN, henR, and hepS are required for heterocyst maturation in Anabaena sp. strain PCC 7120. They presumptively encode two histidine kinases, a response regulator, and a serine/threonine kinase, respectively. To identify relationships between those genes, we compared global patterns of gene expression, at 14 h after nitrogen step-down, in corresponding mutants and in the wild-type strain. Heterocyst envelopes of mutants affected in any of those genes lack a homogeneous, polysaccharide layer. Those of a henR mutant also lack a glycolipid layer. patA, which encodes a positive effector of heterocyst differentiation, was up-regulated in all mutants except the hepK mutant, suggesting that patA expression may be inhibited by products related to heterocyst development. hepS and hepK were up-regulated if mutated and so appear to be negatively autoregulated. HepS and HenR regulated a common set of genes and so appear to belong to one regulatory system. Some nontranscriptional mechanism may account for the observation that henR mutants lack, and hepS mutants possess, a glycolipid layer, even though both mutations down-regulated genes involved in formation of the glycolipid layer. HepK and HepN also affected transcription of a common set of genes and therefore appear to share a regulatory pathway. However, the transcript abundance of other genes differed very significantly from expression in the wild-type strain in either the hepK or hepN mutant while differing very little from wild-type expression in the other of those two mutants. Therefore, hepK and hepN appear to participate also in separate pathways.