Transcriptional dynamics of developmental genes assessed with an FMN-dependent fluorophore in mature heterocysts of Anabaena sp. strain PCC 7120

Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium that differentiates nitrogen-fixing heterocysts when available combined nitrogen is limiting. Growth under diazotrophic conditions results in a mixture of 'new' (recently differentiated) and 'old' (mature) heterocysts. The microoxic environment present in heterocysts makes the interpretation of gene expression using oxygen-dependent fluorophores, including GFP, difficult. The work presented here evaluates the transcriptional dynamics of three developmental genes in mature heterocysts utilizing EcFbFP, a flavin mononucleotide-dependent fluorophore, as the reporter. Expression of both GFP and EcFbFP from the heterologous petE promoter showed that, although GFP and EcFbFP fluoresced in both vegetative cells and new heterocysts, only EcFbFP fluoresced in old heterocysts. A transcriptional fusion of EcFbFP to the late-stage heterocyst-specific nifB promoter displayed continued expression beyond the cessation of GFP fluorescence in heterocysts. Promoter fusions of the master regulator of differentiation, hetR, and its inhibitors, patS and hetN, to GFP and EcFbFP were visualized to determine their role(s) in heterocyst function after morphogenesis. The expression of hetR and hetN was found to persist beyond the completion of development in most heterocysts, whereas patS expression ceased. These data are consistent with a model of heterocyst patterning in which patS is involved in de novo pattern formation, hetN is required for pattern maintenance, and hetR is needed for all stages of development.