IDENTIFICATION OF RHIZOBIUM-SPECIFIC INTERGENIC MOSAIC ELEMENTS WITHIN AN ESSENTIAL 2-COMPONENT REGULATORY SYSTEM OF RHIZOBIUM SPECIES

Analysis of the DNA regions upstream of the phosphoenolpyruvate carbox ykinase gene (pckA) in Rhizobium meliloti and Rhizobium sp, strain NGR 234 identified an open reading frame which was highly homologous to th e Agrobacterium tumefaciens chromosomal virulence gene product ChvI. A second gene product, 500 bp downstream of the chvI-like gene in R. me liloti, was homo!ogous to the A. tumefaciens ChvG protein. The homolog y between the R. meliloti and A. tumefaciens genes was confirmed, beca use the R. meliloti chvI and chvG genes complemented A. tumefaciens ch vI and chvG mutants for growth on complex media. We were unable to con struct chvI or chvG insertion mutants of R, meliloti, whereas mutants carrying insertions outside of these genes were readily obtained. A 10 8-bp repeat element characterized by two large palindromes was identif ied in the chvI and chvG intergenic regions of both Rhizobium species, This element was duplicated in Rhizobium sp. strain NGR234, Another s tructurally similar element with a size of 109 bp was present in R. me liloti but not in Rhizobium sp, strain NGR234. These elements were nam ed rhizobium-specific intergenic mosaic elements (RIMEs), because thei r distribution seems to be limited to members of the family Rhizobiace ae, A homology search in GenBank detected six more copies of the first element (RIME1), all in Rhizobium species, and three extra copies of the second element (RTME2), only in R. meliloti, Southern blot analysi s with a probe specific to RIME1 showed the presence of several copies of the element in the genome of R. meliloti, Rhizobium sp. strain NGR 234, Rhizobium leguminosarum, and Agrobacterium rhizogenes, but none w as present ink. tumefaciens and Bradyrhizobium japonicum.