Generation of a genetic polymorphism in clonal populations of the bacterium Streptomyces ambofaciens: characterization of different mutator states.

In Streptomyces ambofaciens, colony pigmentation is an unstable character. Very unstable mutants selected from twelve wild type (WT) subclones of S. ambofaciens ATCC23877 were investigated. This research showed that the polymorphism in colony pigmentation had distinct features. The first aspect is the coexistence of four types of colonies: pigmented colonies (Pig+), pigment-defective colonies (Pigcol-), pigmented colonies harboring pigment-defective sectors (Pigsec+) or pigment-defective papillae (Pigpap+). The second feature was revealed by the study on Pigpap+ colonies. We showed that WT progeny after 14 days of growth consisted almost totally of Pigpap+ colonies. Pigpap+ colonies were also found to be genetically different from each other. Characterization of twelve colonies presenting more than 20 papillae (Hyperpap colonies) led to the isolation of twelve mutator strains which produced at high frequency Pigcol- and Hyperpap colonies. Each exhibited a specific mutator phenotype and were distinct from each other. Such strains constituted a part of the polymorphism observed in each of the WT progeny and also generated a high variability. Finally, we showed that pigment-defective papillae were mutants and constituted a new form of genetic instability.