The Rhizobium Ieguminosarum biovar viciae nod0 gene can enable a nod€ mutant of Rhizobium leguminosarum biovar trifolii to nodulate vetch

Sciences, University of East Anglia, Norwich NR4 7TJ, UK 2 School of Biological Analysis of the nodulation characteristics of transposon-induced mutants of Rhizobium leguminosarum bv. viciae revealed that nodO and the closely-linked rhi genes contribute t o nodulation of peas (Pisum sativum) and the vetch Vicia hirsuta. Although mutation of nodO alone had no significant effect on nodulation of either legume, a double mutant lacking both nodO and nod€ nodulated both legumes very poorly. Similarly, a double mutant lacking nod€ and either rhiA or rhiB nodulated peas less efficiently than a nod€ mutant. Thus, although mutations affecting only the rhi genes normally have no observed effect on nodulation, these genes do appear to contribute to pea nodulation. When transferred to a wild-type strain of Rhizobium leguminosarum bv. trifolii, neither nodO nor the rhi gene region conferred pea or vetch nodulating ability. However, in a nod€ mutant of R. 1. bv. trifolii, nodO did confer a significant level o f vetch nodulating ability, indicating that the secreted Nod0 protein can play a role in determining legume recognition by R. 1. bv. viciae.