The arginine deiminase pathway in Rhizobium etli: DNA sequence analysis and functional study of the arcABC genes.

Sequence analysis upstream of the Rhizobium etli fixLJ homologous genes revealed the presence of three open reading frames homologous to the arcABC genes of Pseudomonas aeruginosa. The P. aeruginosa arcABC genes code for the enzymes of the arginine deiminase pathway: arginine deiminase, catabolic ornithine carbamoyltransferase (cOTCase), and carbamate kinase. OTCase activities were measured in free-living R. etli cells and in bacteroids isolated from bean nodules. OTCase activity in free-living cells was observed at a different pH optimum than OTCase activity in bacteroids, suggesting the presence of two enzymes with different characteristics and different expression patterns of the corresponding genes. The characteristics of the OTCase isolated from the bacteroids were studied in further detail and were shown to be similar to the properties of the cOTCase of P. aeruginosa. The enzyme has a pH optimum of 6.8 and a molecular mass of approximately 450 kDa, is characterized by a sigmoidal carbamoyl phosphate saturation curve, and exhibits a cooperativity for carbamoyl phosphate. R. etli arcA mutants, with polar effects on arcB and arcC, were constructed by insertion mutagenesis. Bean nodules induced by arcA mutants were still able to fix nitrogen but showed a significantly lower acetylene reduction activity than nodules induced by the wild type. No significant differences in nodule dry weight, plant dry weight, and number of nodules were found between the wild type and the mutants. Determination of the OTCase activity in extracts from bacteroids revealed a strong decrease in activity of this enzyme in the arcA mutant compared to the wild-type strain. Finally, we observed that expression of an R. etli arcA-gusA fusion was strongly induced under anaerobic conditions.