The nature of genetic transformants of Mycoplasma laidlawii

Abstract

Transformation of a streptomycin-resistant marker was effected in the saprophytic mycoplasmas, at frequencies of 5.4 X 10^-6 or 7.6 X 10^-4 'when challenged in broth or by agar overlay, respectively. However, the expression to streptomycin-resistance was unstable. The instability of expression to streptomycin-resistance is attributed to a stable but non-integrated state of the str^r-allele. Furthermore, it is believed that a state of coexistence of the resident str^s-allele and the exogenomic str^r-allele is responsible for the low levels of resistance. Because of this coexistence, complete expression to streptomycin-resistance (50 μg/ml streptomycin) would not be expected. The level of resistance is believed to be dependent upon the quantity of the str^r-allele product(s) synthesized and accumulated by the transformed cell. Evidences indicate that the str^r-allele is stable and functional, although non-replicative, under non-selective conditions, and that it is stable, functional, and replicative under selective conditions. The addition of ribonucleic acids to the transforming DNA enhanced the biological activity of the DNA. The RNA is believed to inhibit DNAse activity in the transforming culture. Because genomic recombinants cannot be distinguished from the spontaneous mutants, in this system of transformation, genetic analyses of the saprophytic mycoplasmas are severely limited.