NODD2 OF RHIZOBIUM SP NGR234 IS INVOLVED IN THE REPRESSION OF THE NODABC OPERON

Transcriptional regulators of the lysR family largely control the expr ession of bacterial symbiotic genes, Rhizobium sp, NGR234 contains at least four members of this family: two resemble nodD, while two others are more closely related to syrM, Part of the extremely broad host ra nge of NGR234 can be attributed to nodD1, although the second gene sha res a high degree of DNA sequence homology with nodD2 of R. fredii USD A191, A nodD2 mutant of NGR234 was constructed by insertional mutagene sis. This mutant (NGR Omega nodD2) was deficient in nitrogen fixation on Vigna unguiculata and induced pseudonodules on Tephrosia vogelii. S everal other host plants were tested, but no correlation could be draw n between the phenotype and nodule morphology. Moreover, nodD2 has a n egative effect on the production of Nod factors: mutation of this gene results in a fivefold increase in Nod factor production, Surprisingly , while the structure of Nod factors from free-living cultures of NGR Omega nodD2 remained unchanged, those from V. unguiculata nodules indu ced by the same strain are non-fucosylated and have a lower degree of oligomerization, In other words, developmental regulation of Nod facto r production is also abolished in this mutant, Competitive RNA hybridi zations, gene fusions and mobility shift assays confirmed that nodD2 d ownregulates expression of the nodABC operon.