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Journal of Bacteriology, December 2008, p. 7645-7654, Vol. 190, No. 23
0021-9193/08/$08.00+0     doi:10.1128/JB.01110-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

HetF and PatA Control Levels of HetR in Anabaena sp. Strain PCC 7120

Department of Microbiology, University of Hawaii, Honolulu, Hawaii 96822

Received 7 August 2008/ Accepted 25 September 2008

Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium that differentiates heterocysts in response to deprivation of combined nitrogen. A hetF deletion strain lacked heterocysts and had aberrant cell morphology. Site-directed mutagenesis of the predicted active-site histidine and cysteine residues of this putative caspase-hemoglobinase fold protease abolished HetF function, supporting the hypothesis that HetF is a protease. Deletion of patA, which is necessary for the formation of most intercalary heterocysts, or hetF resulted in an increase in HetR protein, and extra copies of hetF on a plasmid functionally bypassed the deletion of patA. A hetR-gfp translational fusion expressed from an inducible promoter demonstrated that hetF-dependent downregulation of HetR levels occurs rapidly in vegetative cells, as well as developing heterocysts. "Mosaic" filaments in which only one cell of a filament had a copy of hetR or hetF indicated that hetF is required for differentiation only in cells that will become heterocysts. hetF was required for transcription from a hetR-dependent transcription start point of the hetR promoter and induction of transcription from the patS promoter. The inverse correlation between the level of HetR protein and transcription from hetR-dependent promoters suggests that the transcriptional activity of HetR is regulated by HetF and PatA.

Published ahead of print on 3 October 2008.