RNA expression from a site-specific non-LTR retrotransposon microinjected into Xenopus oocytes

Tx1L is a site-specific non-LTR retrotransposon (NLR) that has been identified in the genome of Xenopus laevis. Using microinjection into Xenopus oocytes, several aspects of RNA expression by these elements were investigated. With constructs carrying various parts of the element we saw no evidence of promoter activity, unlike what has been shown for several other elements of this class. Tx1L transcription was induced by linking a whole element to a promoter that is active in oocytes. Among the RNAs produced, about half had 3' ends located near the end of the element, suggesting that instruction for 3' end formation are encoded in the element or its target. Deletion of the 3' UTR of Tx1L and of surrounding target sequences indicated that these regions are not required for termination or processing of the RNA. PolyA or very A-rich sequences were added at these 3' ends, despite the absence of canonical polyA addition signals. A significant proportion of non-A residues was found in the 3' untemplated tails, and this is reminiscent of non-templated insertions often found at the 3' junction of new genomic copies of some NLRs.