Relative base composition of nuclear RNA transcribed from repetitive and unique DNA in frog embryos

Frog embryo nuclear RNA labeled with undine or guanosine was hybridized to denatured frog or minnow DNA on nitrocellulose filters. Some of the hybrids were treated with RNase and some were untreated. The ratios of RNase-treated/non-RNase-treated filters show that portions of heterogeneous nuclear RNA (hnRNA) transcribed from single-copy DNA are more G-C rich than are the regions transcribed from repetitive DNA. The ratio of the heterologous minnow DNA filter hybridization to the homologous frog DNA hybridization is greater for neurula nuclear RNA transcribed from repetitive DNA than for tailbud nuclear RNA transcribed from repetitive DNA, thus showing that neurula nuclear RNA transcribed from repetitive DNA is more conservative than tailbud nuclear RNA transcribed from repetitive DNA. The ratio of the minnow DNA filter hybridization to the frog DNA filter hybridization is greater for the nuclear RNA transcribed from repetitive DNA of belly regions of tailbud embryos than for the dorsal axial regions, indicating that the nuclear RNA transcribed from repetitive DNA of the belly regions is more conservative. These data are discussed in relation to the idea that restriction of transcription of A-T rich repetitive DNA sequences plays a role in the progressive emergence of less conservative features during early embryonic development.