DAB1: a degenerate retrotransposon-like element from Neurospora crassa.

A repeated DNA sequence in the genome of Neurospora crassa has been identified as a family of degenerate retroelements. Retroelements encode protein sequences with clear homology to the reverse transcriptase, RNase H and endonuclease products of the pol genes common to retroviruses and retrotransposons. These sequence comparisons place the N. crassa element within the gypsy group of retrotransposons, akin to other elements found in filamentous fungi. However, the Neurospora element is defective, as no flanking long terminal repeats (LTRs) could be distinguished and the pol gene homologues contain numerous stop codons as a result of multiple base substitutions. The base composition of the element displays significant under-representation of the dinucleotide CpA, the preferred target site of repeat-induced point mutation (RIP). The genomic sequences exhibit G:C to A:T transitions between copies which are diagnostic of RIP. The degenerate retroelement has accordingly been designated by the acronym dab-1 (dead and buried).