Rates of transposition in Escherichia coli

Abstract

The evolutionary role of transposable elements (TEs) is still highly controversial. Two key parameters, the transposition rate ( u and w , for replicative and non-replicative transposition) and the excision rate ( e ) are fundamental to understanding their evolution and maintenance in populations. We have estimated u , w and e for six families of TEs (including eight members: IS1, IS2, IS3, IS4, IS5, IS30, IS150 and IS186) in Escherichia coli , using a mutation accumulation (MA) experiment. In this experiment, mutations accumulate essentially at the rate at which they appear, during a period of 80 500 (1610 generations × 50 lines) generations, and spontaneous transposition events can be detected. This differs from other experiments in which insertions accumulated under strong selective pressure or over a limited genomic target. We therefore provide new estimates for the spontaneous rates of transposition and excision in E. coli . We observed 25 transposition and three excision events in 50 MA lines, leading to overall rate estimates of u ∼ 1.15 × 10 –5 , w ∼ 4 × 10 −8 and e ∼ 1.08 × 10 −6 (per element, per generation). Furthermore, extensive variation between elements was found, consistent with previous knowledge of the mechanisms and regulation of transposition for the different elements.