Activation of ribosomal RNA synthesis in initiation of Wolffian lens regeneration

Tissue engaged in the transformation of iris into lens, triggered by lens removal in the adult newt ( Triturus viridescens ), was labeled in vitro with 14 C‐uridine, and the sedimentation profiles of RNA synthesized by the tissue were studied in sucrose density gradients. The radioactivity in the 18S and 28S areas, which is insignificant in the normal iris, increases significantly during 2 to 10 days after lens removal, and subsequently retains a high level until the completion of lens regeneration. These data are interpreted as indicating activation of ribosomal RNA (rRNA) synthesis by lens removal, and are discussed in connection with the ultrastructural modification of the nucleoli. Further data from the radioactive profiles are consistent with the notion that the level of control of rRNA production in this system is the transcription of the rRNA precursor rather than the post‐transcriptional processing of the precursor. The activation of rRNA production described in this paper is the earliest event so far detected in this system, preceding activation of DNA replication, cell replication, and dedifferentiation.