RNA synthesis during the DNA synthesis period of the first cell cycle in the root meristem of germinating Vicia faba

The synthesis of RNA during the first partially synchronous DNA synthesis period (S) in the primary root meristem of germinating Vicia faba seeds was studied. Although there was a general temporal correlation between the synthesis of DNA and RNA during the first cell cycle, the bulk of RNA synthesis preceded the bulk of DNA synthesis during G1 and early S. Actinomycin D treatments during S showed that at least part of this RNA synthesis was necessary for the normal rate of DNA replication. It had been shown earlier that protein synthesis is required throughout the first S. Fractionation of rapidly labeling RNA by polyacrylamide gel electrophoresis consistently revealed the synthesis of several distinct RNA species of high specific activity during first S. Actinomycin D reduced the height of all radioactive peaks. The estimated molecular weight of one of the rapidly labeling RNA species (2.4 × 10 6) corresponds to the molecular weights reported for ribosomal precursor RNA in other species of higher plants. Ribosomal RNA was fully processed during first S.