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Journal of Bacteriology, November 2001, p. 6667-6675, Vol. 183, No. 22
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.22.6667-6675.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Identification and Inactivation of Three Group 2 Sigma Factor Genes in Anabaena sp. Strain PCC 7120

Ivan Y. Khudyakov and James W. Golden*

Department of Biology, Texas A & M University, College Station, Texas 77843-3258

Received 18 June 2001/Accepted 22 August 2001

Three new Anabaena sp. strain PCC 7120 genes encoding group 2 alternative sigma factors have been cloned and characterized. Insertional inactivation of sigD, sigE, and sigF genes did not affect growth on nitrate under standard laboratory conditions but did transiently impair the abilities of sigD and sigE mutant strains to establish diazotrophic growth. A sigD sigE double mutant, though proficient in growth on nitrate and still able to differentiate into distinct proheterocysts, was unable to grow diazotrophically due to extensive fragmentation of filaments upon nitrogen deprivation. This double mutant could be complemented by wild-type copies of sigD or sigE, indicating some degree of functional redundancy that can partially mask phenotypes of single gene mutants. However, the sigE gene was required for lysogenic development of the temperate cyanophage A-4L. Several other combinations of double mutations, especially sigE sigF, caused a transient defect in establishing diazotrophic growth, manifested as a strong and prolonged bleaching response to nitrogen deprivation. We found no evidence for developmental regulation of the sigma factor genes. luxAB reporter fusions with sigD, sigE, and sigF all showed slightly reduced expression after induction of heterocyst development by nitrogen stepdown. Phylogenetic analysis of cyanobacterial group 2 sigma factor sequences revealed that they fall into several subgroups. Three morphologically and physiologically distant strains, Anabaena sp. strain PCC 7120, Synechococcus sp. strain PCC 7002, and Synechocystis sp. strain PCC 6803 each contain representatives of four subgroups. Unlike unicellular strains, Anabaena sp. strain PCC 7120 has three additional group 2 sigma factors that cluster in subgroup 2.5b, which is perhaps specific for filamentous or heterocystous cyanobacteria.

* Corresponding author. Mailing address: Department of Biology, Texas A & M University, 3258 TAMU, College Station, TX 77843-3258. Phone: (979) 845-9823. Fax: (979) 845-2891. E-mail: jgolden{at}tamu.edu.

Journal of Bacteriology, November 2001, p. 6667-6675, Vol. 183, No. 22
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.22.6667-6675.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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