Genes involved in diazotrophic growth of Anabaena sp. PCC 7120

Abstract

In Anabaena, in the absence of fixed form of nitrogen, vegetative cells differentiate into nitrogen-fixing heterocysts at semiregular intervals along the filament. Heterocysts can be distinguished from vegetative cells microscopically by their larger size and thicker cell envelope. In order to create microaerophilic conditions for the activity of nitrogenase, they have two additional layers of envelope made of polysaccharides and glycolipids (34). The glycolipid layer, which is the innermost of the two, provides a hydrophobic barrier against the entry of oxygen (46, 48). The exterior polysaccharide layer is thought to preserve the integrity of the glycolipid layer. Genes necessary for the production and loca1ization of both layers have been found and pathways for their synthesis have been proposed (13, 20). Once a microaerophilic environment has been created inside the heterocysts, they fix atmospheric nitrogen and transport it to vegetative cells and in return receive a source of reductant required for fixation from vegetative cells (42). The pattern of heterocysts along a filament is determined by the interplay of positive and negative acting regulatory factors, and approximately 12 hours after the removal of fixed nitrogen, select cells have committed to termina1 differentiation into heterocysts (31, 50, 56). PatS and HetR appear to be the two central factors that control differentiation and pattern formation. HetR is the master regulator and has both DNA-binding and protease activity (7, 21, 57). It displays positive autoreglation and expression of hetR is induced in proheterocysts prior to commitment to differentiation (5). In order to identify the genes involved in diazotrophic growth and differentiation of heterocysts by Anabaena, a genetic screen was conducted to isolate mutants incapable of growth in the absence of fixed nitrogen. Interruption of the coding region of fraG, the gene upstream of hetR, by a transposon resulted in a fragmentation mutant that was unable to grow in the absence of a fixed source of nitrogen. The predicted protein is similar to permeases and is necessary for filament integrity and maturation of heterocysts to the point of glycolipid layer formation. The enzymes that synthesize and remodel the peptidoglycan are generally known as penicillin binding proteins (PBPs). In E. coli, 12 PBPs have so far been identified, of which only PBPs la and 1b are essential for cell viability (54). In Anabaena, peptidoglycan is present inner to the glycolipid layer. A pbpB mutant of Anabaena was incapable of fixing atmospheric nitrogen under aerobic conditions (28). This mutant in the presence of fixed nitrogen did not show any significant difference in the phenotype with respect to that of the wild type. But, in the absence of a fixed form of nitrogen, filaments were yellow, short and twisted. Vegetative cells were unequal in size and shape. Heterocysts were distorted with thin envelopes and with no cyanophycin granules at the poles. In the genetic screen described, a pbp6 mutant that was unable to fix atmospheric nitrogen under aerobic conditions was obtained. The predicted protein consists of both the transglycosidase as well as 1ranspeptidase domains that might be involved in the formation of peptidoglycan in Anabaena.