Effect of growth rate, nutrient limitation and succinate on expression of TOL pathway enzymes in response to m-xylene in chemostat cultures of Pseudomonas putida (pWW0).

Abstract

Previous studies have shown that expression of the toluene and m- and p-xylene degradation pathway in Pseudomonas putida (pWW0) is subject to catabolite repression by succinate. We report here that the expression level of the upper part of this so-called TOL pathway in cells grown in chemostat culture is strongly influenced by nutrient limitation when m-xylene is the sole carbon and energy source. The benzylalcohol dehydrogenase (BADH) levels in cells that are growth-limited by anabolic processes [sulphate (S)-, phosphate (P)- or nitrogen (N)-limiting conditions] were 3-12% of those in cells growing under oxygen limitation (when catabolism limits growth). BADH levels under S-, P- and N-limitation were further decreased (three- to fivefold) when succinate was supplied in addition to m-xylene. Levels of the meta-cleavage pathway enzyme catechol 2,3-dioxygenase were less affected by the growth conditions but the general pattern was similar. Dilution rate also influenced the expression of the TOL pathway: BADH levels gradually decreased with increasing dilution rates, from 1250 mU (mg protein)-1 at D = 0.05 h-1 under m-xylene limitation to 290 mU (mg protein)-1 at D = 0.58 h-1 (non-limited growth). BADH levels were shown to be proportional to the specific affinity whole cells for m-xylene. It may, therefore, be expected that natural degradation rates are adversely affected by anabolic nutrient limitations, especially at relatively low concentrations of the xenobiotic compound.