Characterization and Mutational Analysis of nodHPQ Genes of Rhizobium sp. Strain N33. Jean Cloutier. Recherche en sciences de la vie et de la sante, Pavilion Charles-Eugene Marchand, Universita Laval, Quebec (Quebec), Canada G1K 7P4. Serge Laberge(2), Yves Castonguay(2), and Hani Antoun(1). (1) Recherche en sciences de la vie et de la sante, Pavilion Charles-Eugene Marchand, Universita Laval, Quebec (Quebec), Canada G1K 7P4; (2) Centre de recherche et de developpement sur les sols et les grandes cultures, Agriculture et Agroalimentaire Canada, 2560, boul. Hochelaga, Sainte-Foy (Quebec), Canada G1V2J3. MPMI 9:720-728. Accepted 5 August 1996. Copyright 1996 The American Phytopathological Society.

We have shown, by sequencing the nodulation gene region of Rhizobium sp. strain N33 previously isolated from the Canadian high arctic, that the nodHPQ genes are located in a 4.8-kb region downstream of nodBCIJ. The open reading frames of nodHPQ are 747, 906, and 1941 nucleo-tides long, respectively. The strain N33 genome contains one copy of nodH and two copies of nodPQ that are homologous to those genes in Rhizobium meliloti. Tn5 insertions in the nodHPQ genes of strain N33 did not affect the formation of nodules on the two homologous hosts, Astragalus cicer and Onobrychis viciifolia. Since strain N33 contains the nodBCIJHPQ genes and the recently sequenced nodAFEG genes, we looked for similar host range with R. meliloti. Strain N33 and R. meliloti strains A2 and RCR2011 were shown to induce the formation of root nodules on plants of O. viciifolia. However, strain N33, compared with R. meliloti strains, was able to elicit a few, white, empty, root nodules on Medicago sativa. R. meliloti strains, compared with strain N33, were shown to induce only few nodules containing bacteria on A. cicer. Induction of nod genes transcription in strain N33 was shown to be induced by a variety of flavonoid compounds that are different from those inducing nod genes from R. meliloti.