SUMMARY: Five spontaneous erythromycin resistance mutations arising independently in a pneumococcal strain have been studied. Three distinct levels of resistance are represented by these mutants (0.1, 1.0, and 10.0 μg. erythromycin per ml.). The mutations conferring resistance can be transferred to the sensitive parental strain through transforming DNA preparations. The transfer is discrete, in that the full level of resistance of the donor strain is always conferred upon the recipient. The length of time required for phenotypic expression of a mutation acquired by transformation depends on the particular marker.

A mutation in a given strain may either be replaced by or combine with a different mutation transferred from a donor DNA preparation. In the case of combination, the DNA of the recombinant is capable of transferring each of the mutations as well as the entire complex of mutations possessed by the recombinant. The frequency of transfer of the complex demonstrates the degree of linkage of the separable mutations. A group of mutations in a given recombinant strain may either display antagonistic, synergistic or non-synergistic effects on the phenotype.

Reverse mutations towards erythromycin-sensitivity generally involve alteration at the originally mutated sites, or at very closely linked sites.


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