Figure 7
Model for protein-primed reverse transcription by the pFOXC-reverse transcriptase (RT). Transcription of the pFOXC plasmid DNA molecules produces full-length RNAs that appear to function as both mRNAs for the synthesis of the RT and as templates for (-) strand cDNA synthesis [6]. Transcripts of pFOXC3 terminate in approximately three pentameric repeats, whereas transcripts of pFOXC1 terminate in approximately four copies of a 3 bp sequence (the 3' terminus of in vitro RNA used in this study is shown). Following production of the plasmid-encoded RT, deoxynucleotidylation occurs with the covalent addition of dAMP to a tyrosine residue of the 60 kDa pFOXC3-RT, followed by incorporation of deoxyguanosine monophosphate (dGMP) and a third nucleotide. Deoxynucleotidylation of the pFOXC1-RT results in the addition of thymidine monophosphate (TMP) to the RT, followed by one or more deoxynucleotide monophosphates (dNMPs) (a second TMP is shown). The resulting RT-(dNMP)n complex would have complementarity to the corresponding terminal repeat. Based on studies of protein-primed DNA elements, the model predicts that the complex anneals to the penultimate 3' repeat of the template (shown for pFOXC1 only). Following the synthesis of a unit-length repeat, the RT-(dNMP)n complex undergoes a slideback and is repositioned opposite the terminal repeat. The nascent cDNA is elongated via reverse transcription of the template by the 5'-linked RT or by a separate RT recruited to the complex. The model could also accommodate an increase in the number of repeats, depending on the number of slideback events that occur.