MECHANISM OF OSMOTICALLY REGULATED N-ACETYLGLUTAMINYLGLUTAMINE AMIDE PRODUCTION IN RHIZOBIUM-MELILOTI

Rhizobium meliloti adapts to environments of high osmolarity by accumu lating glutamate, trehalose, and the dipeptide N-acetylglutaminylgluta mine amide (NAGGN) intracellularly. In this study, the mechanism of NA GGN production and accumulation was examined. NAGGN was produced in os motically shocked cultures after a lag period of more than one hour, a nd NAGGN was undetectable in cultures treated with chloramphenicol, in dicating that genetic induction is required for NAGGN accumulation. In vitro radiolabeling experiments demonstrated that the peptide synthes is step in NAGGN production did not occur ribosomally. Rather, it was catalyzed by an ATP-dependent enzyme that appeared to be both induced by high osmolarity and activated by K+. Also, a mutant analysis sugges ted that NAGGN may be partly responsible for the osmotic tolerance obs erved in R. meliloti. 36% of mutants that were characterized as osmoti cally sensitive compared to the parent strain, were also found to cont ain reduced levels of. NAGGN. The phenomenon of osmolyte accumulation as it relates to adaptation to other environmental stresses is discuss ed.