Initiation specificity of the poly(cytidylic acid)-dependent Qbeta replicase activity

The initiation specificity of RNA synthesis catalysed by the poly(C)-dependent Qbeta replicase activity was investigated with various synthetic ribopolymers as template. It was found that the initiation efficiency of a series of oligo(C) with various chain lengths is proportional to the template size. Synthetic riboheteropolymers containing cytidylic acid were accepted as templates only if they contained at their 3' end a cytidylic acid sequence of more than 5 nucleotides. Such an oligocytidylate sequence served also as an initiator sequence for copying the non-cytidylic-acid-containing part of the heterotemplate. RNA synthesis always began with the incorporation of GTP, even if the 3'-terminating nucleotide of the template was not cytidylic acid.