Mutagenesis of a transformable strain of pneumococcus by -methyl-′-nitro--nitrosoguanidine produced several mutants resistant to ampicillin. One of these was resistant to 0·1 μg. ampicillin/ml., but a purified solution of its DNA transformed the sensitive parent to three different levels of resistance at frequencies compatible with single, double and triple transformants. The higher levels of resistance were dependent on the presence of the genes conferring the lower resistances. Furthermore, transformants to the higher levels of resistance can be obtained at single or double frequency by using as recipient the strain already possessing the genes conferring the lower resistances. The expression times of the ampicillin genes are short (approximately 25 min.), but the actual times were difficult to determine since there was a delay of some 15 to 20 min. before the ampicillin exerted an effect on the sensitive strain under these conditions.


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