Identification of rhizobium genes that are induced under symbiotic growth conditions in the bean symbiont strain R. etli CE144 which lacks the symbiotic plasmid.

Rhizobium are a unique group of bacteria classified by their ability to initiate the formation of a novel organ, the nodule, on legume plants. The Rhizobium-legume symbiosis has been labeled as one of the most important plant-bacteria interactions (Puhler, 1998). The large symbiotic plasmids, pSyms, have been widely recognized as the carrier of essential symbiotic genes. Here, we sought to identify genes that are induced during simulated symbiotic conditions (low pH and flavonoids) that are not located on the pSym. Two methods, conventional vector cloning and transposon mutagenesis (containing a gusA reporter gene), were employed of which the latter - was successful in obtaining insertion mutants in R. etli CE144 (pSymderivative of R. etli CE3). Four mutants exhibited greater than 2-fold activity compared to normal in the -glucuronidase assay. Normal was defined as -glucuronidase activity under non-induced conditions. Sequence analysis revealed that insertions in the four mutants were a putative oxidoreductase, ABC transporter, hydrolase, and glutamine synthetase II (GSII). Due to high sequence similarity to GSII, this mutant was analyzed for a distinct phenotype and the observations are reported.