Tuesday, May 3, 2011
Caulobacter crescentus is a Gram-negative, oligotrophic bacterium with the ability to metabolize a variety of organic compounds, including plant polysaccharides, to obtain carbon source for growth. C. crescentus has been proven to utilize cellobiose, xylan, or pectin as its sole carbon source. Comparative 2-D gel electrophoresis analysis using whole cell lysates of cells grown in cellobiose/glucose or xylan/xylose as the sole carbon source shows differential protein expression, indicating altered metabolism in different carbon sources. GC-MS analysis of the C. crescentus growth medium containing cellobiose or xylan demonstrates extracellular generation of glucose and xylose respectively, suggesting the expression of extracellular or membrane-bound glycosyl hydrolases for the breakdown of cellobiose and xylan. Genomic analysis of C. crescentus shows coding for over 80 proteins, including several glycoside hydrolases, with signal sequences for the Twin arginine (Tat) - pathway mediated functional protein translocation across membranes. Proteomic identification of TonB-dependent transporters (TBDT) suggests that monosaccharides generated by the action of glycoside hydrolases are subsequently transported across the outer membrane into the periplasm via membrane transporters. Several glycoside hydrolases, TBDTs, and ABC sugar transporters are found in single transcriptional units in the C. crescentus genome, suggesting coordinated actions of cellobiose/xylan degradation and import of the corresponding monosaccharide across outer and inner membranes into cytoplasm. Knockout mutants of C. crescentus with functionally disrupted glycoside hydrolases, TBDTs found in these transcriptional units were generated to investigate the role of glycoside hydrolases and TBDTs in the generation and transport of monosaccharides across the outer membrane into the periplasm.