Determinant of resistance to kanamycin in Streptomycin rimosus: amplification in the chromosome and reversible genetic instability

The study on the kanamycin resistance determinant (Kanr) in an oxytetracycline--producing strain of S. rimosus showed that it was capable of amplifying in the chromosome during selection for increasing the antibiotic resistance level. The amplification of the DNA fragment with a molecular weight of 10.3 MDa containing Kanr amounted to 300 copies per genome, which resulted in a more than 1000-fold increase in kanamycin resistance level. Cloning of the Kanr determinant on plasmid SLP1.2 in S. lividans strain 66 was performed. In Streptomyces lividans strain 66 the Kanr determinant preserved the capacity for amplification in the hybrid plasmid pSU10 integrated into the chromosome. The Kanr determinant in the strains of S. rimosus and S. lividans was characterized by transfers Kanr in equilibrium Kans with a frequency of 1 X 10(-3). It was shown that the mutation in S. lividans strain 66 resulting in phenotype Kans was not connected with the structural Kanr gene on plasmid pSU10 but was localized on the chromosome. Phenotype Kans was promoted by a decrease in the number of the copies of the regulatory genetic element designated RES1. The reverse to phenotype Kanr might be due to one of the following events: amplification to the initial level of RES1 and amplification up to 200 copies per the genome of the hybrid plasmid pSU10 containing the Kanr determinant. Amplification of the Kanr determinant with preserved initial level of RES1 element resulted in a more than 1000 times increase in the resistance level.