Physiological modulation of the sigma54-dependent promoter Pu of the TOL plasmid: negative regulatory role of the TurA protein of Pseudomonas putida in the response to suboptimal growth temperatures

The Pseudomonas putida sigma54-dependent Pu promoter of the TOL upper operon, involved in toluene utilization, represents an optimal reference model to investigate the complexity of the regulatory networks for aromatic substances. Indeed, Pu activation in the presence of aromatic effectors is finely tuned in response to several factors connected to cell physiology and environmental physiochemical conditions. The current model for Pu activation includes several details about the molecular switch involved in toluene response. However, the molecular mechanisms of the fine physiological regulation remain to be fully understood. We identified, using a reverse-genetic approach, a small Pu-binding protein, TurA, and we present evidence that TurA-Pu DNA interactions could participate in direct negative modulation of Pu activity both in vivo and in vitro. Furthermore, we present evidence that TurA-mediated repression of Pu activity is further increased at suboptimal growth temperatures and that induction of TOL upper operon at suboptimal temperatures is growth limiting. Our results show that TurA cellular concentrations are growth-phase and temperature regulated. We suggest that the role of TurA could be limitation of the expression of the toluene-degrading enzymes under suboptimal growth conditions to prevent toxic effects. Our in silico analysis indicated that TurA and, possibly, the other members of the MvaT family are structurally related to the nucleoid-associated protein H-NS of Escherichia coli. It can be suggested that the members of the MvaT family have roles in Pseudomonas species analogous to H-NS-like proteins in other Gram-negative species as global modulators of gene expression.