Template-determined, Variable Rate of RNA Chain Elongation

Q beta replicase polymerizes MDV-1 RNA at a markedly variable rate. Electrophoretic analyses of partially synthesized strands showed that a few of the elongation intermediates are much more abundant than others, reflecting a variable rate of chain elongation. Our data suggest that at a relatively small number of specific sites in the sequence of this RNA, the progress of the replicase is temporarily interrupted, and then resumes spontaneously, with a finite probability. Since the time spent between these pause sites is negligible compared with the time spent at pause sites, the mean time of chain elongation is well approximated by the sum of the mean times spent at each pause site. Nucleotide sequence analysis of the most prominent elongation intermediates indicated that they all have the potential to form a 3' terminal hairpin structure. This suggests that the marked variability in the rate of chain elongation is due to the formation of terminal hairpins in the product strand, or the reformation of hairpins in the template strand. A survey of the literature shows that this phenomenon occurs with most, if not all, nucleic acid polymerases. Structure-induced pauses may play a role in the regulation of nucleic acid synthesis.