Chemoreceptor-based signaling is a major bacterial signal transduction mechanism. Escherichia coli, the traditional model, has five chemoreceptors. Recent genome analyses have shown that many bacteria have a much higher number of chemoreceptors. Pseudomonas putida KT2440 is an alternative model that has 27 chemoreceptors and the cognate chemoeffector is known for many of them. Here, we address the question whether and which factors modulate chemoreceptor gene expression. We report reverse transcriptase quantitative PCR measurements of all KT2440 chemoreceptor genes. Transcript levels of individual chemoreceptors differed largely, namely up to 174-fold between the most and least abundant. The cognate chemoeffectors had three different effects on the expression of their chemoreceptor genes. In some cases, the respective chemoeffectors, shown previously to be C- and/or N-sources, induced the expression. In contrast, for the two inorganic phosphate sensing chemoreceptors, the chemoeffector caused dramatic reduction in expression. For four other receptors, including the three TCA cycle intermediate sensing receptors, the chemoeffector did not cause any significant alterations. In addition, a significant number of receptors were induced in minimal growth medium and in the stationary phase. We show here that environmental cues determine largely chemoreceptor expression. This work will serve as reference for analogous studies in other bacteria.
Keywords: Chemoreceptor; Chemosensory pathway; Chemotaxis; Gene expression; Pseudomonas.