REGULATION OF RIBOSOMAL RNA SYNTHESIS DURING TAIL REGRESSION IN ORGAN CULTURE.

Tail tips of early prometamorphic larvae of Xenopus laevis were organ-cultured in Stearns' solution. Triiodothyronine induced a number of responses characteristic of metamorphosis, such as regression, decrease in DNA and RNA content, and increase in H-uridine incorporation into RNA. The synthesis of ribosomal RNAs and its degradation in regressing tail tips were studied. The results obtained are: (1) the rate of transcription of ribosomal RNA, as well as the over-all rate of total RNA synthesis, does not differ between control tail tips and treated ones: (2) the 40S precursor RNA to ribosomal RNAs is processed at a higher rate in treated tips than in controls during the late period of treatment; (3) ribosomal RNAs synthesized (as well as transfer RNA) become greatly susceptible to some degradative activity in the regressing tail tips during the late period of treatment, and this may account for the observed decrease in RNA content; and (4) no evidence was obtained for the 'asymmetric' processing of the precursor molecule which has been suggested to occur in the tail muscle of prometamorphic tadpoles of Xenopus laevis.