Role of hetF and patU3 in the regulation of heterocyst development in Anabaena sp. Strain pcc 7120

Abstract

A central paradigm in developmental biology concerns the differentiation of cells despite the fundamental sameness of the genetic complement shared by all cells in the organism. The filamentous cyanobacterium Anabaena sp. strain PCC 7120 is an ideal model system for the study of development. In response to nitrogen deprivation, Anabaena differentiates nitrogen-fixing heterocyst cells in a periodic pattern. Anabaena research has practical applications in human health, sustainable agriculture, and biofuel production. This study aimed to characterize novel interactions to refine the current understanding of Anabaena development. Investigations were performed to elucidate the hetF-dependent activation of differentiation. A novel genetic regulatory network involving hetF, a CHF class protease, and the negative regulator patU3 and other developmental genes was identified. A component of the hetZ-patU5-patU3 gene cluster, PatU3 was shown to directly interact with HetZ, another activator of differentiation. Genetic epistasis analysis determined that PatU3 suppressed positive regulation by HetZ and HetR. These negative feedback loops explain the elevated HetR-GFP concentrations in hetF-dependent strains despite the paradoxical absence of heterocysts. The HetF-dependent pathway may act as a control point prior to commitment to the heterocyst cell fate. Lateral inhibition by PatS and HetN, which both contain the same RGSGR pentapeptide sequence, involves regulation by HetR. The unique domains present in the structure of HetR may relate to its activity as a transcriptional activator. In this study, genetic and cytological approaches were used to identify residues in HetR necessary for interaction with PatS, HetN and RGSGR. A related investigation demonstrated that the RGSGR-pentapeptide derived from HetN directs pattern formation by direct cytoplasmic exchange. Lastly, protein phosphorylation plays a prominent role in varied biological processes. The PP2C-type protein phosphatase All1758 was characterized in this study. The corresponding all1758 gene is controlled by the developmental genes ntcA and hetR. All1758 affects later stages of differentiation and may represent a critical link between cell growth, cell division and morphogenesis by controlling putative sigma factor regulators (antisigma factors and antisigma factor agonists) and cell division genes (including FtsZ and MinCE). Taken together, these findings support additional regulatory mechanisms necessary for proper Anabaena development.