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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

March 1997 , Volume 10 , Number  2
Pages  280 - 289

Michael F. Cohen and John C. Meeks

Section of Microbiology, Division of Biological Sciences, University of California, Davis 95616 U.S.A.

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Accepted 13 November 1996.

Transposon-generated mutant strain UCD 328 of Nostoc punctiforme strain ATCC 29133 has a phenotype of an increased sensitivity to a hormogonium-inducing factor exuded by a symbiotic plant partner, Anthoceros punctatus, and an initial increased hormogonium-dependent infection of the plant. Sequence analysis showed that the transposition site in strain UCD 328 lies within a 1,251-bp open reading frame (ORF), designated hrmA, that displays no significant similarity to known database sequences. 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-dependent oxidoreductases. Strains having insertional mutations in hrmU or hrmA reproduce the strain UCD 328 phenotype. Transcriptional fusions of luxAB to hrmU or hrmA show an 8- to 10-fold peak increase in luciferase 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 extract was deduced to be between 2.0 to 3.4 kb from the translational start of hrmU. A multicopy plasmid that contains hrmUA within a 6.2-kb fragment conferred an increased infection phenotype on wild-type N. punctiforme 29133. This plasmid and another plasmid containing 4.4 kb of DNA 5′ of the transposition site prevented extract-dependent induction of hrmA-luxAB transcription in strain UCD 328, implicating titration of some trans-activator(s) by the cloned fragments. We hypothesize a role for hrmUA in the inhibition of hormogonium formation by the metabolism of an unknown hormogonium-regulating metabolite.

Additional keywords: hormogonium differentiation, multicopy inhibition, Nostoc sp. strain ATCC 29133, omega-npt cassette, plant-cyanobacterial signaling, symbiotic cyanobacterium.

© 1997 The American Phytopathological Society