Integration and nonrandom mutation of a plasma membrane proton ATPase gene fragment within the Bs1 retroelement of maize.

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Abstract
Retrotransposons are a class of mobile DNA sequences in eukaryotes that transpose through a reverse-transcribed RNA intermediate. Retrotransposons containing long terminal repeats have many of the attributes of retroviruses in animals but have not been previously observed to acquire a portion of a cellular gene as RNA tumor viruses do with oncogenes. We have found homology to plasma membrane proton ATPase genes within the Bs1 retrotransposon of maize, and this homology led us to clone the maize plasma membrane proton ATPase gene, which we have named Mha1. The sequence of Mha1 confirmed that 654 bp of this ATPase gene are present in Bs1; this segment represents the last amino acid of exon 4, all of exons 5 to 9, and part of exon 10. All introns have been removed from this acquired DNA, whereas 81 single base pair substitutions and a deletion of 183 bp in Bs1 differentiate these contiguous segments. The secondary mutations led to fewer changes in the derived Bs1 protein sequence than predicted for neutral events, suggesting that the acquired Mha1 DNA performs a selected function within Bs1. These data indicate that a retrotransposon can incorporate and transmit a portion of a standard nuclear gene transcript within its genetic material. Alternatively, these results suggest that Bs1 may represent a defective version of a plant retrovirus.