INTEGRATION AND NONRANDOM MUTATION OF A PLASMA-MEMBRANE PROTON ATPASEGENE FRAGMENT WITHIN THE BS1 RETROELEMENT OF MAIZE

Retrotransposons are a class of mobile DNA sequences in eukaryotes tha t transpose through a reverse-transcribed RNA intermediate. Retrotrans posons containing long terminal repeats have many of the attributes of retroviruses in animals but have not been previously observed to acqu ire a portion of a cellular gene as RNA tumor viruses do with oncogene s. We have found homology to plasma membrane proton ATPase genes withi n 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 p resent 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 remove d from this acquired DNA, whereas 81 single base pail. substitutions a nd a deletion of 183 bp in Bs1 differentiate these contiguous segments . The secondary mutations led to fewer changes in the derived Bs1 prot ein sequence than predicted for neutral events, suggesting that the ac quired Mha1 DNA performs a selected function within Bs1. These data in dicate that a retrotransposon can incorporate and transmit a portion o f a standard nuclear gene transcript within its genetic material. Alte rnatively, these results suggest that Bs1 may represent a defective ve rsion of a plant retrovirus.