| Abstract: | It has been previously shown that newly synthesized nuclear low molecular weight RNA species C and D are first detected in the cytoplasm for a few minutes before they are finally found in the nucleus. The following are some of the observations made in the present study, regarding the formation of C and D RNA: (1) The 5′ end cap ribose methylation of the C RNA precursor is complete in its cytoplasmic stage; the internal ribose methylation of the precursor seems to be completed about the time of its apparent transition from cytoplasm to nucleus. (2) The few nucleotides lost from the D RNA precursor during maturation seem to be excised sometime near its apparent cytoplasmic → nuclear transition. Newly synthesized C RNA also appears to lose some of its non-conserved nucleotides about the time of that transition, while the other extra nucleotides are lost later, in the nucleus. (3) The maturation of C and D RNA is inhibited early during suppression of protein synthesis by cycloheximide, while their synthesis is not. (4) The cytoplasmic precursors of C and D RNA are not associated with ribonucleoprotein particles as large as those reported for mature C and D RNA, although they do not appear to be free in the cytoplasm. (5) When the cellular UTP pool is depleted by exposure of the cells to amino sugars, and the synthesis of C, D, and other RNA species decreases, the level of3H]uridine labeling of C and D RNA increases, while that of 4 S and 5 S RNA does not. These data are compatible with the existence of more than one nuclear UTP pool. |
