Regulation of gene expression in psychrophilic micro organisms: molecular aspects and biotechnological applications

Abstract

Two-component systems are the most common mechanism of transmembrane signal transduction in bacteria and are involved in many cellular processes. They control the expression of genes for nutrient acquisition, virulence, antibiotic resistance, and numerous other processes in diverse bacteria. These systems are often used by soil bacteria that have to adapt to frequent changes in nutritional availability and in environmental conditions to regulate their gene expression. By contrast, bacteria that live in stable niches (symbionts of aphids, extremophiles or some marine hydrocarbon-degrading bacteria, etc.) seem to have less regulatory systems. Recently, a functionally active two- component system (named PhMAlRS) probably involved in the of expression regulation of a C4-dicarboxylate transporter system was identified in the Antarctic bacterium Pseudoalteromonas haloplanktis TAC125. Our data demonstrated that the expression of the C4-dicarboxylate transporter system (consisting of genes coding for an outer membrane porin and a periplasmic transporter, renamed Phdct A and B, respectively) is induced by the presence of L-malate under the control of the PhMalRS two-component regulatory system. The DNA region located upstream the PhdctA porin gene was further investigated; two different transcriptional start sites and two different promoter sequences were defined, one of which is a typical σ core promoter. A 53 bp inverted repeat sequence located 77 bp upstream the distal transcriptional start site was also identified. Functional analysis of this element showed that the inverted repeat sequence was specifically recognised by protein extracts of PhTAC125 only when the cells were grown in the presence of L-malate, suggesting a putative regulatory role for this DNA region. We can speculate that this DNA sequence represents the cis-acting region requested for the binding of transcriptional regulatory proteins leading to the induction of the PhdctAB operon expression. These results demonstrated the occurrence of a fine transcriptional mechanism that regulates the expression of a C4-dicarboxylate transporter operon in Pseudoalteromonas haloplanktis TAC125. In the presence of L-malate, the PhMalRS regulatory system controls the expression of the PhdctA and B genes very likely through binding of a regulatory protein to the cis-acting region located upstream the outer membrane porin gene. The interaction of the regulatory protein with this DNA sequence probably strengthens the binding of RNA polymerase on the σ core promoter sequence and activates the transcription of downstream genes responsible for L-malate uptake.