Inhibition of glutamine synthetase II expression by the product of the gstI gene

We report the identification of a previously unrecognized gene that is invo lved in the regulation of the Rhizobium leguminosarum glnII (glutamine synt hetase II) gene. This gene, which is situated immediately upstream of glnII , was identified by means of a deletion/complementation analysis performed in the heterologous background of Klebsiella pneumoniae. It has been design ated gstI (glutamine synthetase translational Inhibitor) because, when a co mplete version of gstI is present, it is possible to detect glnII-specific mRNA, but neither GSII activity nor GSII protein. The gstI gene encodes a s mall (63 amino acids) protein, which acts in cis or in trans with respect t o glnII and is transcribed divergently with respect to glnII from a promote r that was found to be strongly repressed by the nitrogen transcriptional r egulator NtrC. A mutated version of GstI lacking the last 14 amino acids co mpletely lost its capacity to repress glnII expression. Our results indicat e that gstI mediates the translation inhibition of glnII mRNA and, based on in silico analyses, a mechanism for GstI action is proposed.