Metabolism of high molecular weight, polydisperse, rapidly labeled nuclear RNA in rat liver

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Abstract
Studies of the metabolism of rat liver RNA showed the existence of two species of rapidly labeled nuclear RNA: a 45 S preribosomal type of nucleolar origin, and a 6–50 S polydisperse RNA of chromosomal origin. The kinetics of labeling with orotic acid-14C and the nature of the latter RNA have been investigated. The following findings are reported, (1) This RNA is composed of at least four main classes of RNA having sedimentation coefficients of approximately 45, 35, 24, and 18 S. (2) Except for the 18 S class which seems to be an end product, the three other classes have a rapid turnover and do not appear to leave the nucleus. (3) Base analysis after 32P incorporation indicates that these four classes of RNA have a similar base composition with a G+C/A + U ratio in the range of 0.98–1.07, which resembles DNA more closely than ribosomal RNA. (4) The 6–50 S polydisperse RNA has a different metabolism than that of the 45 S preribosomal type and there is no precursor-to-product relationship between these two species of RNA.

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