Adaptive reversion of a frameshift mutation in Escherichia coli by simple base deletions in homopolymeric runs

Abstract

Spontaneous mutations are thought to occur primarily in growing cells. However, spontaneous mutations also arise in nutritionally deprived cells, and in some cases this process appears to be adaptive. Here it is reported that when a Lac- strain of Escherichia coli is under selection for lactose use, the spectrum of Lac+ mutations that arises is different, and simpler, than that arising without selection. Mutations appearing during selection were mainly one-base deletions in runs of iterated bases. Similar mutations occurring in repetitive DNA elements are associated with a variety of human hereditary diseases and are increased in cells that cannot correct heteroduplex DNA.

Fig. 1

The target for reverting mutations in the lacI33-lacZ allele. Sites at which −1-bp deletion mutations were found more than once are indicated in bold. Numbering is as in the Ecolac sequence in Gen-Bank up to base pair 1144, but the extra C at base pairs 1036 to 1038, which creates the lacI33 allele, is not numbered. The coding strand is shown. The lacI-lacZ gene fusion (13) eliminates the last five residues of LacI, all of the lac promoter and operator, and the first 23 residues (or 24 if the initiating methionine is included) of LacZ (base pairs 1145 to 1356 of the Ecolac sequence). The fusion protein is transcribed from the mutant lacI^q promoter. The frameshift mutation at base pairs 1036 to 1038 was induced by ICR191 (14). This mutation creates a stop codon at base pair 1145 (base pair 1358 of the Ecolac sequence), and a −1 frameshift mutation upstream of base pair 1016 would create a stop codon at base pair 1016. Thus, to revert lacI33, the reading frame must be restored between base pairs 1016 and 1145.