WHAT MAKES GRANDE1 RETROTRANSPOSON DIFFERENT

Grande1 elements constitute a family of Ty3 retrotransposons present i n the Zea genus in more than 1000 copies in Zea diploperennis and maiz e. The sequences of three Grande1 flanking regions, two from Z. diplop erennis and one from maize, reveal transposable elements as insertion targets, suggesting a preferential integration of Grandel elements int o other transposable elements. These retrotransposons are remarkable f or their large size of around 14 kb, which is a consequence of a very large 3' region of more than 7 kb. Atypical entities within this regio n are two arrays of unrelated tandem repeats with potential stable ste m-loop structures. A large portion of the same region is occupied by O RFs, although only ORF23, whose function is unknown, is presumably tra nscribed in antisense orientation to the reverse transcriptase ORE Onl y ORF23 has a codon usage similar to the one tabulated for highly-expr essed maize genes. Correspondingly, the transcript of 900 b that hybri dizes with ORF23 probes is found in all the maize tissues explored. Th is is despite the high level of methylation in the DNA of Grande1. Gen omic RNA has not been detected in any tissue or situation studied, pro bably reflecting a non-functional retrotransposon. The origin of ORF23 and the remainder 3' region might be due to a transduction event.