A minor fraction of basic fibroblast growth factor mRNA is deaminated in Xenopus stage VI and matured oocytes

Adenosine deaminases that act on RNA (ADARs) convert adenosine to inosine i n double-stranded regions of RNA. ADAR activity is in the nucleus in Xenopu s laevis stage VI oocytes, and released into the cytoplasm at oocyte matura tion. We previously demonstrated that a cytoplasmic double-stranded RNA (ds RNA) binding factor(s), cyto-dsRBP, protects microinjected dsRNA from the A DAR released at maturation. Here we describe experiments to determine wheth er an endogenous dsRNA, the duplex formed between sense and antisense trans cripts of basic fibroblast growth factor (bFGF), is protected in a similar manner. Consistent with the presence of cyto-dsRBP, we observed that the ma jority of bFGF RNA was not deaminated, before or after maturation. However, a minor fraction of the bFGF RNA was deaminated whether the RNA was isolat ed from stage VI oocytes or matured oocytes. Since ADAR activity is in the nucleus in stage VI oocytes, our results suggest that a fraction of the bFG F RNAs are hybridized in the nucleus and are ADAR substrates. Adenosine dea minations result in A-to-G changes in cDNAs, so we quantified the fraction of modified molecules using restriction-enzyme assays of RT-PCR products. C aveats due to recombination during RT-PCR are discussed.