EPIMUTATION OF REPEATED GENES IN ASCOBOLUS-IMMERSUS

Ascobolus immersus artificial gene repeats were shown previously to be subject premeiotically to both cytosine methylation and inactivation. We studied sexual progenies of strains harbouring two wild type copies of the endogenous met2 gene lying either in tandem array or at ectopic unlinked positions, by (i) investigating the methylation status, (ii) searching for mutations and (iii) analysing the inheritance of inactivation both in mitotic and sexual offspring. 100% of the 'tandem' progeny and 64% of the 'ectopic' progeny had methylated repeats and displayed gene inactivation. Similar methylation patterns involving all or most of the cytosine residues within the repeats were observed in both arrangements. The inactivated met2 copies were totally devoid of mutation, as deduced from: (i) extensive restriction site analysis and DNA sequencing; (ii) the finding that all the Met- derivatives tested reverted to prototrophy in selective conditions; and (iii) the finding that an inactivated copy of met2 stripped of its methylation through amplification in Escherichia coli regained activity when reintroduced in A. immersus. In the absence of selection, gene silencing and methylation were faithfully maintained through mitotic divisions and through five successive sexual cycles. Altogether, these data show the epimutational nature of this methylation induced premeiotically (MIP) process.