Identification of genes and gene products involved in the nodulation of peas by Rhizobium leguminosarum

Abstract

Mutant strains of Rhizobium leguminosarum have been isolated in which the transposon Tn5 was inserted into the cloned nodulation genes. The mutant strains were examined for their ability to induce nodules and to induce normal root hair curling on peas. Five distinct classes of nod mutations were identified on the basis of the phenotypes of the various mutant strains, and of the mapped locations of the nod:: Tn5 alleles. Three classes of mutants (I, II and III) were defective in nodulation and two classes (IV and V) were delayed in nodule appearance. Mutations of classes I, II and III types affected root hair curling and a subcloned 6.6 kb fragment of DNA containing this region conferred the ability to induce root hair curling upon a Rhizobium strain deleted of its nod genes. Analysis of protein products formed after in vitro transcription and translation of the plasmid DNA corresponding to the class I, II, III and IV regions indicated that proteins of molecular weights 48,000, 45,000, 36,000 and 34,000 were formed. A comparison of protein products formed by equivalent plasmids carrying nod:: Tn5 alleles indicated that the class I mutations blocked the synthesis of the 45,000 molecular weight peptide and that the two products of molecular weights 36,000 and 34,000 were absent from the class II mutants. It is concluded that at least five nod genes are involved in nodulation and the polypeptide products of some of these genes have been identified.