New data on excisions of Mu from E. coli MCS2 cast doubt on directed mutation hypothesis.

According to the directed mutation hypothesis, certain mutations in bacteria occur more frequently in environments in which the resulting phenotype is selectively favoured than in non-selective environments. This hypothesis therefore challenges the fundamental tenet that mutations occur spontaneously, irrespective of effects on the organism's fitness. One purported case of directed mutation is the excision of a Mu sequence from Escherichia coli strain MCS2 in minimal lactose-arabinose medium. Here, we show that this case can be more simply explained by an accelerated rate of excision mutation in response to non-specific physiological stresses of starvation and by slight growth of MCS2 on minimal lactose-arabinose medium.