A Study Of Genes Involved In The Formation Of The Heterocyst Spacing Pattern In The Cyanobacterium Nostoc Punctiforme And Their Roles In The Symbiotic Growth State PDF Download

The goal of this research was to understand regulation of heterocyst differentiation in Anabaena sp. strain PCC 7120 by characterizing regulatory genes for heterocyst formation and their mutants. Anabaena is a filamentous cyanobacterium that forms specialized cells for nitrogen fixation, called heterocysts, which differentiate from vegetative cells at intervals of 10--12 cells. Two genes, patS and hetN, are known to suppress the differentiation of vegetative cells into heterocysts for establishing a de novo pattern and maintaining a pattern of heterocysts along the filament. A mutant, UHM100, was created to study the function of both genes by deleting patS and making expression of hetN conditional. This study has established that PatS and HetN are members of two separate heterocyst suppression pathways. In absence of nitrogen, inactivation of both patS and hetN increases heterocyst differentiation to nearly 100%, giving rise to a phenotype called 'multiple contiguous heterocysts' (Mch). UHM100 has an Mch phenotype even in the presence of combined nitrogen, which usually suppresses heterocyst differentiation. In absence of both patS and hetN, the expression of hetR, a master regulator of heterocyst differentiation, was observed in ~55% cells and was asynchronous. The distribution of heterocysts next to a vegetative cell in UHM 100 was found to be nonrandom. These results suggest that besides PatS and HetN, there are other factors that influence pattern formation in Anabaena PCC 7120. A heterocyst-deficient (Hef) spontaneous mutant, NSM6, was isolated from UHM 100. A novel gene, alr9018, from the Anabaena Epsilon plasmid complemented NSM6 and restored the Mch phenotype of this mutant. Transconjugants of Anabaena PCC 7120 containing the cloned alr9018 gene fixed 50% more N2 than PCC 7120, suggesting that multiple copies of alr9018 enhance heterocyst development. This is the first report showing that the Epsilon plasmid of Anabaena PCC 7120 contains genes involved in heterocyst differentiation. Expression of alr9018 was observed in both vegetative cells and heterocysts. Similar to alr9018, hetR could also restore the Mch phenotype in NSM6, suggesting functional similarity between a1r9018 and hetR. The Alr9018 protein contains an NTPase domain, which is a characteristic of proteins involved in signal transduction.