The m-xylene biodegradation capacity of Pseudomonas putida mt-2 is submitted to adaptation to abiotic stresses: evidence from expression profiling of xyl genes.

The effect of archetypal environmental stresses on expression of the catabolic xyl genes of the TOL plasmid pWW0 of the m-xylene degrading strain Pseudomonas putida mt-2 has been investigated. To this end, a subgenomic DNA chip was employed which included structural and regulatory DNA sequences of the TOL pathway along with selected descriptors of specific physiological conditions. Cells were separately exposed to m-xylene under various oxygen tensions, temperatures and nitrogen sources as well as situations of DNA damage, oxidative stress, carbon and iron starvation, respiratory chain damage, and contact with arsenic, but at doses which did not cause a gross effect on growth or cell viability. The incidence of each stress class was categorized through the corresponding descriptors in the chip in respect to the relative output of xyl transcripts. While most of the stresses downregulated the m-xylene biodegradation-related genes, some uncouplers of the respiratory chain (azide) and small doses of arsenate appeared to stimulate their expression. The replacement of NH4+ by NO3- as N source augmented expression of the TOL cistrons also. We subsequently subjected P. putida mt-2 cells to the multiple abiotic stress brought about by exposure to crude tar from the 2002 oil spill of the Prestige tanker, which embraces a complex mixture of hydrocarbons. The resulting expression profile of xyl genes and stress-responding markers over time suggested that adaptation to external insults precedes any significant expression of the catabolic genes. The consequences of this hierarchy of responses for microbial biodegradation in situ are discussed.