Involvement of a Spiroplasma citri plasmid in the erythromycin-resistance transfer.

An erythromycin-resistant strain (M4 Er-1) was selected from Spiroplasma citri M4+. The transfer by transformation of the erythromycin-resistance character to the erythromycin-sensitive S. citri strain R8A2+ was studied. Transfer became effective and reproducible when cells were treated with alkali cations plus polyethylene glycol. Comparison of the efficiency of transformation of the erythromycin-sensitive strain S. citri R8A2+ by total and extrachromosomal DNA purified from the erythromycin-resistant strain M4 Er-1 showed that the plasmid pM42 was able to transfer the erythromycin-resistance. pM42 was mapped with restriction endonucleases and found to be related to the pMH1 plasmid previously isolated from S. citri MH. Hybridization analysis of DNA from sensitive and resistant strains has shown that a sequence from pM42, analogous to a sequence from pMH1, was integrated at a specific locus in the chromosome of the erythromycin-resistant cells, i.e., of the transformed R8A2 cells and of the spontaneous mutant M4 Er-1 strain.