Global GacA-steered control of cyanide and exoprotease production in Pseudomonas fluorescens involves specific ribosome binding sites

The conserved two-component regulatory system GacS/GacA determines the expr ession of extracellular products and Virulence factors in a variety of Gram -negative bacteria. In the biocontrol strain CHA0 of Pseudomonas fluorescen s, the response regulator GacA is essential for the synthesis of extracellu lar protease (AprA) and secondary metabolites including hydrogen cyanide. G acA was found to exert its control on the hydrogen cyanide biosynthetic gen es (hcnABC) and on the aprA gene indirectly via a posttranscriptional mecha nism. Expression of a translational hcnA'-'lacZ fusion was GacA-dependent w hereas a transcriptional hcnA-lacZ fusion was not. A distinct recognition s ite overlapping with the ribosome binding site appears to be primordial for GacA-steered regulation. GacA-dependence could be conferred to the Escheri chia coli lacZ mRNA by a 3-bp substitution in the ribosome binding site. Th e gene coding for the global translational repressor RsmA of P. fluorescens was cloned. RsmA overexpression mimicked partial loss of GacA function and involved the same recognition site, suggesting that RsmA is a downstream r egulatory element of the GacA control cascade. Mutational inactivation of t he chromosomal rsmA gene partially suppressed a gad defect. Thus, a central , GacA-dependent switch from primary to secondary metabolism may operate at the level of translation.