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Journal of Bacteriology, September 2005, p. 6031-6038, Vol. 187, No. 17
0021-9193/05/$08.00+0     doi:10.1128/JB.187.17.6031-6038.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Heterocyst-Specific Excision of the Anabaena sp. Strain PCC 7120 hupL Element Requires xisC

Claudio D. Carrasco,1,{dagger} Scott D. Holliday,1 Alfred Hansel,2,{ddagger} Peter Lindblad,2 and James W. Golden1*

Department of Biology, Texas A&M University, College Station, Texas 77843-3258,1 Department of Physiological Botany, EBC, Uppsala University, Uppsala SE-752 36, Sweden2

Received 18 April 2005/ Accepted 3 June 2005

In nitrogen-limiting conditions, approximately 10% of the vegetative cells in filaments of the cyanobacterium Anabaena (Nostoc) sp. strain PCC 7120 differentiate into nitrogen-fixing heterocysts. During the late stages of heterocyst differentiation, three DNA elements, each embedded within an open reading frame, are programmed to excise from the chromosome by site-specific recombination. The DNA elements are named after the genes that they interrupt: nifD, fdxN, and hupL. The nifD and fdxN elements each contain a gene, xisA or xisF, respectively, that encodes the site-specific recombinase required for programmed excision of the element. Here, we show that the xisC gene (alr0677), which is present at one end of the 9,435-bp hupL element, is required for excision of the hupL element. A strain in which the xisC gene was inactivated showed no detectable excision of the hupL element. hupL encodes the large subunit of uptake hydrogenase. The xisC mutant forms heterocysts and grows diazotrophically, but unlike the wild type, it evolved hydrogen gas under nitrogen-fixing conditions. Overexpression of xisC from a plasmid in a wild-type background caused a low level of hupL rearrangement even in nitrogen-replete conditions. Expression of xisC in Escherichia coli was sufficient to produce rearrangement of an artificial substrate plasmid bearing the hupL element recombination sites. Sequence analysis indicated that XisC is a divergent member of the phage integrase family of recombinases. Site-directed mutagenesis of xisC showed that the XisC recombinase has functional similarity to the phage integrase family.

* 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.

{dagger} Present address: Ambion RNA Diagnostic, 2130 Woodward St., Suite 200, Austin, TX 78744-1832.

{ddagger} Present address: Molecular and Cellular Biophysics Research Unit, Medical Faculty of the Friedrich-Schiller University, Drackendorfer Strasse 1, D-07747 Jena, Germany.

Journal of Bacteriology, September 2005, p. 6031-6038, Vol. 187, No. 17
0021-9193/05/$08.00+0     doi:10.1128/JB.187.17.6031-6038.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.





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