A hotspot for transition mutations in the rIIB gene of bacteriophage T4. I. The extent of the hotspot.

We have previously demonstrated that the sequence 5'TGGCAA 3' located at codons 32-33 of the rIIB gene of bacteriophage T4 is a hotspot for transition mutations (Nelson et al. 1981). Here I report the properties of the same TGGCAA sequence introduced into the gene at codons 11-12. The sequence is highly mutable in both locations, suggesting that its high mutability is due to features of the TGGCAA itself and is not dependent on the immediate juxtaposition of additional external sequences. Within this sequence, at either location, only the transition at the central G:C pair frequently arises spontaneously or by 2-aminopurine or ethylmethane sulfonate mutagenesis. However, the 3' G:C pair, in addition, is highly mutable after nitrous acid or hydroxylamine treatment. This suggests that, within the TGGCAA sequence, there are two hotspots which are targeted by different mutagens.