[Homology-dependent inactivation of LTR retrotransposons in genomes of Aspergillus fumigatus and A. nidulans].

Repeat-induced point mutation (RIP) is the most intriguing among the known mechanisms of repeated sequences inactivation because of its ability to produce irreversible mutation of repeated DNA. Discovered for the first time in Neurospora crassa, RIP is characterized by C:G to T:A transitions in duplicated sequences. The mechanisms and distribution of RIP are still purely investigated. Mobile elements are a common target for the processes which lead to homology-dependent silencing because of their ability to propagate themselves. We have done comparative analysis of LTR retrotransposons in genomic scale from genomes of two aspergilli fungi--Aspergillus funmigatus and A. nidulans, based on several copies we reconstructed "de-RIP" retroelements. Investigations of frequencies of CpG, CpA and TpG sites, which are potential targets for mutagenesis, showed the much lower frequencies of these sites in mobile elements in comparison with structural genes. LTR retrotransposons from A. fumigatus and A. nidulans have different ratio of types of substitutions. Our analysis indicates that two investigated fungi have or had the RIP-like processes for repeated sequences inactivation, in various modes. Whereas in A. fumigatus the context for mutagenesis consists of both CpG and CpA sites, in A. nidulans inactivation seems to proceed only on CpG dinucleotides. The present investigation gives a theoretical background for planning of experimental studying of RIP inactivation in aspergilli.