REVERSAL OF MU-GEM2TS-INDUCED MUTATIONS

Mutations induced by the integration of a Mu gem2ts mutant prophage ca n revert at frequencies around 1 X 10(-6), more than 10(4)-fold higher than that obtained with Mu wild-type. Several aspects characterize Mu gem2ts precise excision: (i) the phage transposase is not involved; ( ii) the RecA protein is not necessary; and (iii) revertants remain lys ogenic with the prophage inserted elsewhere in the host genome. In add ition, prophage re-integration seems to be non-randomly distributed, w hereas Mu insertion into the host genome is a transposition event with out any sequence specificity. In this paper, we describe that the site of re-integration somehow depends on the original site of insertion. Two alternative models are proposed to explain the strong correlation between donor and receptor sites.