Proteome analysis of the model microsymbiont Sinorhizobium meliloti: Isolation and characterisation of novel proteins

University, Canberra City, Sinorhizobium meliloti is an agriculturally and ecologically important microbe due to its capacity to establish nitrogen-fi xing symbiosis with plant legumes. Two-dimensional University, Canberra Cit y, gel electrophoresis of total cellular protein was used to establish a pr oteome reference map for the model microsymbiont Sinorhizobium meliloti str ain 1021. The extent of changes in the gene expression of cells grown in a defined medium at different growth phases was established. After examinatio n of over 2000 resolved protein spots, a minimum of 52 reproducible changes in protein expression levels were detected when early exponential phase ce lls were compared to late exponential phase cells. In contrast, induction o f nodulation gene expression by the addition of the flavonoid luteolin to c ells did not result in detectable changes in protein expression at either e arly or late exponential phase. N-terminal microsequencing of eighteen unkn own constitutive proteins plus four proteins, induced or up-regulated in la te exponential phase cells, allowed the identification of proteins not prev iously described in rhizobia. These included an amide-binding protein, a pu tative hydrolase of the glyoxalase II protein family, a nucleoside diphosph ate kinase, and a 5'-nucleotidase. N-terminal microsequencing was also valu able in revealing N-terminal post-translational processing and assigning a subcellular location to the analysed protein. Proteome analysis will provid e a powerful analytical tool to complement the sequencing of the genome of strain 1021.