Messenger RNA in HeLa cells: Kinetics of formation and decay

The polyadenylic acid-containing messenger RNA fraction of HeLa cells was measured by its affinity for oligedeoxythymidylate cellulose. Both the kinetics of initial labeling and the decay after a brief pulse of incorporation were examined. The kinetics of decay are complex, but can be approximated by assuming two populations; a short-lived species with a half-life of seven hours and a long-lived component with a half-life of 24 hours. It is estimated that the short-lived material comprises 33% of total cellular mRNA, while the relatively stable species amounts to 67% of the steady-state mRNA content. The two mRNA components with different decay times were observed simultaneously in the same cell population by measuring decay of 24-hour old mRNA labeled with ¹⁴C and RNA briefly labeled with ³H. The old mRNA had only a 24-hour decay component, while the new mRNA was biphasic. The decay of old and new mRNA was also observed after RNA synthesis was inhibited with actinomycin. Again, old mRNA decayed more slowly than recently labeled material. However, both decay times are significantly shorter in the presence of actinomycin and correspond to half-lives of approximately 4 and 12 hours. There is a small but significant difference in sedimentation distribution of new and old mRNA, the old mRNA sedimenting more slowly than new material, suggesting that the more stable species has a lower average molecular weight. The steady-state content of mRNA in HeLa cells amounts to 5.5% of the ribosomal RNA, or more than twice the amount of messenger RNA estimated to be on hemoglobin-synthesizing polyribosomes.