Control of transformation competence in Azotobacter vinelandii by nitrogen catabolite derepression

Abstract

Vinelandii (ATCC 12837) became competent to be transformed by exogenous DNA towards the end of exponential growth. Competence in wild-type and nitrogenase auxotrophic (nif-) strains was repressed by the addition of ammonium salts or urea to the transformation medium. Transformation of wild-type cells and nif- strains was optimal on N-free or N-limiting medium, respectively. Transformation of wild-type cells also was enhanced when the transformation medium had low molybdate content. During the development of competence, N was growth limiting, and C (glucose) was in excess. C source shift-down was not effective in inducing competence. Shifting glucose-grown wild-type cells to medium containing 0.2% .beta.-hydroxybutyrate initiated encystment and also induced competence. The addition of glucose to this medium blocked encystment and early competence induction and reduced the transformation frequency to the basal level. Cyclic AMP induced competence in wild-type N2-fixing cells and increased the transformation frequency 1000-fold over the basal level. Exogenous cyclic AMP did not reverse N repression of competence in ammonia-grown wild-type or nif- strains.