DNA binding properties of the HrmR protein of Nostoc punctiforme responsible for transcriptional regulation of genes involved in the differentiation of hormogonia

Summary Nostoc punctiformeis an example of a filamentous cyanobacterium that is capable of differentiating non‐growing cells that constitute gliding filaments termed hormogonia. These gliding filaments serve in short distance dispersal and as infective units in establishing a symbiosis with plants, such as the bryophyteAnthoceros punctatus. Mutants ofN.punctiformeexist which show elevated levels of initial infection ofA.punctatusas a consequence of repeated cycles of hormogonium differentiation. Such mutations occur within thehrmAandhrmUgenes. Further characterization of thehrmlocus revealed several genes with an organizational and predicted protein sequence similarity to genes of heterotrophic bacteria that are involved in hexuronic acid metabolism. Genes in theN. punctiformelocus are transcribed in response to the presence of a plant extract containing hormogonium‐repressing factors. A predicted transcriptional repressor encoded in the locus, HrmR,was shown herein to be a specific DNA binding protein that regulates the transcription of its own gene and that ofhrmE, a nearby gene. The ability of HrmR to bind DNA was abolished upon addition of either galacturonate or lysate from specifically inducedN.punctiformecells, implying that thein vivo HrmR binding activity is modulated via an internal compound, most likely a sugar molecule.