Fig. 1. Influence of trans-acting factors on the types of alterations occurring in tetranucleotide repeat tracts in Hi. Alterations in the 5′AGTC repeat tracts of phase-variant colonies were classified as either insertions or deletions of the repeat units (i.e. an increase or decrease, respectively, in the number of repeat units) and as changes of one, two or more repeat units. The numbers of variants in each class were expressed as a percentage of the total number examined. Relevant genotypes and total number of variants examined for (in parentheses) each strain are indicated below each column. (A) ON-to-OFF switching. Parental colonies contained 38 (wt and most of the mutants) or 35 (ΔpolI) 5′AGTC repeats. Data for the ΔrecA mutants is the combined data for parental colonies containing either 41 or 35 5′AGTC repeats. (B) OFF-to-ON switching in which the +1 reading frame produces a Mod–LacZ fusion protein. Parental colonies contained, in most cases, 37 5′AGTC repeats. Data for the wt and ΔpolI strains is the combined data for parental colonies containing either 37 or 31 repeats [in the former case, the data were reported previously (De Bolle et al., 2000)].