A HORMOGONIUM REGULATING LOCUS, HRMUA, OF THE CYANOBACTERIUM NOSTOC-PUNCTIFORME STRAIN ATCC-29133 AND ITS RESPONSE TO AN EXTRACT OF A SYMBIOTIC PLANT PARTNER ANTHOCEROS-PUNCTATUS

Transposon-generated mutant strain UCD 328 of Nostoc punctiforme strai n ATCC 29133 has a phenotype of an increased sensitivity to a hormogon ium-inducing factor exuded by a symbiotic plant partner, Anthoceros pu nctatus, and an initial increased hormogonium-dependent infection of t he plant. Sequence analysis showed that the transposition site in stra in UCD 328 lies within a 1,251-bp open reading frame (ORF), designated hrmA, that displays no significant similarity to known database seque nces. A second, 837-bp ORF (hrmU) ends 2 bp 5' from the start of hrmA and has the signature sequences belonging to a family of NAD(P)H-depen dent oxidoreductases, Strains having insertional mutations in hrmU or hrmA reproduce the strain UCD 328 phenotype, Transcriptional fusions o f luxAB to hrmU or hrmA show an 8- to 10-fold peak increase in lucifer ase activity 13 to 20 h after the start of incubation in the presence of an aqueous extract of A. punctatus. A promoter induced by the extra ct was deduced to be between 2.0 to 3.4 kb from the translational star t of hrmU. A multicopy plasmid that contains hrmUA within a 6.2-kb fra gment conferred an increased infection phenotype on wild-type N. punct iforme 29133. This plasmid and another plasmid containing 11.4 kb of D NA 5' of the transposition site prevented extract-dependent induction of hrmA-luxAB transcription in strain UCD 328, implicating titration o f some trans-activator(s) by the cloned fragments. We hypothesize a ro le for hrmUA in the inhibition of hormogonium formation by the metabol ism of an unknown hormogonium-regulating metabolite.