KINETICS OF INCORPORATION OF URIDINE-C14 INTO L CELL RNA.

Five components have been isolated from L cells by a combination of phenol extraction procedures and sedimentation analysis through sucrose gradients. These components are identified by their sedimentation rates. The 50S and 40S components are derived from the nucleus, the 32S and 18S from ribosomal RNA, and the 4S fraction is the soluble RNA of the cell. L cells were supplied with uridine-C(14) under steady-state conditions and the rate of uptake of C(14) into each component was measured. Analysis of the results suggests that the delay in entry of C(14) into ribosomal RNA is occasioned by two sequential precursors and that 50S and 40S RNA meet the kinetic requirements for these precursors. 4S RNA seems to contain two components that label at different rates.