Directed mutation: are mutations always spontaneous and random?

Abstract

The term directed mutation describes those mutation occurring as a specific response to stress. "Escherichia coli" strain K12, lac-(i-,z-) was used in this study. Overnight cultures were plated on MacConkey's basal media or MacConkey's basal media plus lactose. Revertants to lac+ which occur within two days of plating are considered to be due to phenotypic lag. Revertants produced after two days, which can be observed as red papillae on large white colonies, are considered to be the result of mutations occurring after plating. The data shows an increase in the number of lac+ revertants growing in the presence of lactose. It has been suggested that this observed increase is due to transcriptional bias. This hypothesis suggests that during transcription, the DNA of the lac operon becomes single-stranded and more vunerable to mutation. In the strain we are using there is a mutation in the lac I gene that allows constitutive transcription of the operon and thus the DNA in this region is continually in a single-stranded state. However, with this strain, we find an increase in revertants only in the presence of lactose suggesting that another mechanism may be operating in this operon.