Evidence of post-transcriptional regulation of U6 small nuclear RNA.

Mechanisms regulating the intracellular level of endogenous U6 small nuclear RNA were studied by transient transfection of ectopic U6 gene constructs into immortalized normal and malignant human cell lines. Transfection and expression of a modified U6 gene containing native promoter, capping, and termination sequences but lacking all highly conserved internal spliceosome sequences produced dose-dependent effects on endogenous U6 gene expression. At low transfection doses, no significant changes in endogenous U6 RNA levels or half-life were noted. However, as the dose of the transfected gene and its expression increased, native U6 RNA levels dramatically decreased in association with an apparent decrease in U6 RNA half-life. Down-regulation of native U6 RNA levels was transient, with recovery noted within 48-96 h in conjunction with declining expression of the ectopic gene. These modulatory effects appeared specific to endogenous U6 transcripts, because no changes were noted in 7sk, U1, U3, or 5S RNA levels or half-lives. Transfection with an unmodified U6 gene did not alter total U6 transcript levels but did produce a similar dose-dependent decrease in U6 RNA half-life. These studies suggest a hitherto unrecognized U6-specific intracellular regulatory mechanism, through which over-accumulation of U6 small nuclear RNA is prevented.