PatS Peptide Inhibits Heterocyst Development and Influences Pattern Formation in <i>Anabaena</i>

Abstract

The developmental pattern of nitrogen-fixing het erocysts along filaments of multicellular cyano bacteria provides an interesting example of prokaryotic developmental biology (Wolk et al. 1994). To best serve the needs of the organism, the cyanobacterium must regulate the frequency and position of heterocysts along the filament to bal ance the supply of products from photosynthesis and nitrogen fixation. The pattern of heterocysts varies among different cyanobacterial strains and can be altered by the environment as well as in symbiotic associations. The study by Golden and Yoon (1998) reported on the genetics and molecu lar biology of heterocyst pattern formation in Anabaena sp. strain PCC 7120. The differentiation of a vegetative cell into a heterocyst is estimated to involve the regulation of hundreds of genes. Heterocysts possess a modified photosynthetic capability, grossly modified internal membranes, a specialised thickened cell envelope, an altered physiology adapted to support nitrogen fixation, large polar cyanophycin granules for ni trogen storage and the enzymes for nitrogen fix ation and uptake hydrogenase activity (Wolk et al. 1994). Heterocyst differentiation in many strains is accompanied by programmed site-specific DNA rearrangements that result in the excision of DNA elements from within heterocyst-specific genes (Golden 1997). In Anabaena strain PCC 7120 het erocysts, DNA elements are excised from two genes involved in nitrogen fixation, nifD andfdxN, and an uptake hydrogenase gene, hupL (Carrasco et al. 1998). A variety of genetic and molecular tools are available for studies of Anabaena strain PCC 7120 (Wolk et al. 1994). The isolation of DNA, RNA and protein for a variety of procedures and assays is now routine. Although electroporation can be used to introduce DNA molecules into Anabaena strain PCC 7120, conjugation with Escherichia coli is the preferred method of genetic manipulation. Shuttle vectors can be used to introduce and main tain genes for complementation and expression by their own or by heterologous promoters. Non replicating conjugal plasniids are used to obtain single or double recombination with homologous sequences on the chromosome for gene inactiva tion or manipulation. Gene fusions with lacZ, luxAB, and gfp reporter genes have been used to follow gene expression under a variety of environ mental, physiological and developmental con ditions.