Metabolism of U6 RNA species in nonirradiated and UV-irradiated mammalian cells.

We observed a series of rapidly labeled U6 RNA bands, which were hybrid selected with U6 DNA, in nonirradiated human cells. The electrophoretic mobility of these bands in denaturing gels was lower than that of the known mature U6 RNA species, and was equivalent to transcripts up to approximately 7 nucleotides longer. These multiple U6 RNA species lost their label during a chase without a proportional increase in radioactivity in the known mature U6 RNA, which suggests that a substantial fraction is not processed into the major mature U6 RNA. During a label chase, the multiple U6 RNA bands appeared first in the cytoplasmic fraction and later in nuclei. One of the major rapidly labeled U6 RNA bands had the electrophoretic mobility of an RNA species one nucleotide shorter than the known mature U6 RNA. UV light induced a UV dose-dependent, preferential disappearance of recently synthesized molecules of the U6 RNA species of higher gel electrophoretic mobility, including the known mature U6 RNA. Since this effect was seen in cells pulse-labeled immediately before or after irradiation, it suggests that UV radiation induces the specific degradation of the electrophoretically faster moving species of U6 RNA, which are apparently shorter chains. The effect of UV light was RNA species-specific, was not seen in molecules synthesized long (e.g., 22 hr) before irradiation, and occurred in human and mouse cells.