Marker discrimination and mutagen-induced alterations in pneumococcal transformation.

Nitrous acid-induced and hydroxylamine-induced chemical alterations in transforming DNA result in the loss of biological activity and in mutagenesis. The function responsible for the discrimination between high efficiency and low efficiency markers in pneumococcal transformation, and for the elimination of a substantial fraction of spontaneously occurring mutational events, does not appear to act on integrated DNA carrying these chemical alterations. The chemical modifications result in mutations that are evident among bacteria transformed with the treated DNA. Fusidic acid-resistant mutants isolated in this way have been shown to be predominantly of the low efficiency class. We have previously reported that most mutations of spontaneous origin occurring in this locus are of the high efficiency class, and have suggested that discrimination results from the elimination of specific classes of base pair mismatches that occur as intermediates in transformation. It would appear that the base pair mismatches most effectively eliminated in the discriminating strain of pneumococcus are those of the A:C and G:T type and that the immediate product of transformation with mutagenized DNA involves intermediates that are not recognized as A:C or G:T by the discrimination system.