NATIVE DNA REPEATS AND METHYLATION IN ASCOBOLUS

We identified two classes of native dispersed DNA repeats in the Ascob olus genome. the first class consisted of several kilobase long, methy lated repeats. These repeats, named Mars (methylated Ascobolus repeate d sequences), fell in one family of LINE-like elements and in three fa milies of LTR-containing retrotransposable elements. The methylation f eatures of Mars elements were those expected if they were natural targ ets for the MIP (methylation induced premeiotically) previously discov ered in Ascobolus. The second class consisted of short repeats, simila r to 100 bp long, corresponding to 5S rRNA and tRNA genes. As expected from their size, which was too small to allow MIP to occur, they were unmethylated, as were 26 kb of unique sequences tested. These observa tions are consistent with the hypothesis that MIP is targeted at natur al DNA repeats and constitutes a defensive process against the detrime ntal consequences of the spreading of mobile elements throughout the g enome. The 9 kb tandem repeats harbouring the 28S, 18S and 5.8S rRNA g enes displayed methylation features suggesting that rDNA methylation p roceeds through a process other than MIP.