FraG is necessary for filament integrity and heterocyst maturation in the cyanobacterium Anabaena sp strain PCC 7120

Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium that differentiates nitrogen-fixing heterocysts when fixed nitrogen becomes growth limiting in the medium. The gene alr2338 (designated fraG herein), located immediately upstream of the master regulator of differentiation hetR, was identified in a genetic screen for mutants unable to grow diazotrophically. Filaments with a mutation in fraG were unable to fix nitrogen or synthesize heterocyst-specific glycolipids, and they fragmented initially to approximately nine cells in length at 24 h after induction of heterocyst development and eventually became unicellular. The fragmentation phenotype could be duplicated in the presence of fixed nitrogen when differentiation of heterocysts was elicited by overexpression of hetR, suggesting that a defect in differentiation, and not a lack of fixed nitrogen in the medium, was the more direct cause of fragmentation. An intact fraG gene was necessary for differentiation of mature heterocysts, but was not required for proper pattern formation, as indicated by a normal pattern of expression of hetR in a fraG mutant. A transcriptional GFP reporter fusion indicated that the level of expression of fraG was low in vegetative cells in both nitrogen-replete and nitrogen-free media, and was induced in heterocysts. fraG appears to play a role in filament integrity and differentiation of proheterocysts into mature heterocysts.