Differential pH-induced proteins in Rhizobium tropici CIAT 899 and Rhizobium etli CIAT 611

Response to pH shift of Rhizobium tropici CIAT 899 and Rhizobium etli CIAT 611, reacting differently to reduced pH in their nodulation efficiency, was studied by two-dimensional gel electrophoretic analysis of soluble proteins from cells labeled with [35S]methionine. After a pH shift from pH 6.8 to 5.2, the electrophoretic patterns of both strains revealed marked differences of protein synthesis. In R. tropici CIAT 899, five polypeptides were increased, whereas four others were decreased in their synthesis following a pH shift, while in R. etli CIAT 611, ten polypeptides, distinct from those in R. tropici 899, were increased relative to pH 6.8, and the synthesis of eight polypeptides decreased after the shift. Two pH shift induced proteins (PipA and PipB) of R. tropici CIAT 899 were excised from two-dimensional gels and were subjected to microsequencing, revealing similarity of PipA to enzymes likely to be involved in the synthesis of exopolysaccharides (EPS).