The Saccharomyces cerevisiae hyperrecombination mutation hpr1Δ results in instability of sequences between direct repeats that is dependent on transcription of the repeat. Here it is shown that the HPR1 gene also functions in plasmid stability in the presence of destabilizing transcription elongation. In the hpr1Δ mutant, plasmid instability results from unchecked transcription elongation, which can be suppressed by a strong transcription terminator. The plasmid system has been used to examine in vivo aspects of transcription in the absence of Hpr1p. Nuclear run-on studies suggest that there is an increased RNA polymerase II density in the hpr1Δ mutant strain, but this is not accompanied by an increase in accumulation of cytoplasmic mRNA. Suppression of plasmid instability in hpr1Δ can also be achieved by high-copy expression of the RNA splicing factor SUB2, which has recently been proposed to function in mRNA export, in addition to its role in pre-mRNA splicing. High-copy-number SUB2 expression is accompanied by an increase in message accumulation from the plasmid, suggesting that the mechanism of suppression by Sub2p involves the formation of mature mRNA. Models for the role of Hpr1p in mature mRNA formation and the cause of plasmid instability in the absence of the Hpr1 protein are discussed.
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