The human minisatellites MS1, MS32, MS205 and CEB1 integrated into the yeast genome exhibit different degrees of mitotic instability but are all stabilised by RAD27.

The yeast Rad27 protein is homologous to mammalian Fen1 and is involved in the processing of replication intermediates. Enhanced instability of various artificial repetitive DNA sequences in RAD27-deficient yeast strains has been observed previously and shown to involve preferentially expansion mutations. In the present investigation, we characterised the mitotic instability of alleles of the naturally occurring human minisatellites MS1, MS32, MS205 and CEB1 and the modified MS1 alleles containing more highly homogeneous repeat regions than the original alleles. These minisatellites demonstrated more pronounced instability in rad27 Delta strains, with increases in the frequencies of both expansion and contraction mutants. In RAD27 strains, MS32 and MS205 were relatively stable, while MS1 and CEB1 were unstable, indicating that the effect of RAD27 on stability is influenced by intrinsic properties of the repeat array. This conclusion received further support from the remarkably high frequency of length-mutants observed for the modified allele of MS1. Thus, our findings emphasise the importance of: (1) comparing results obtained with various naturally occurring minisatellites and (2) manipulating their sequences in attempts to understand the molecular basis for mitotic stability/instability of minisatellite DNA.