Roles of the Host Polypeptides in Qp RNA Replication

Initiation of transcription of favored templates by Qi3 replicase requires a much lower GTP concentration than does transcription of templates that are selected against. Although the enzyme requires a high GTP concentration to initiate transcription of Qp RNA, the presence of the host factor substantially reduces the GTP concentration requirement. In addition, the Qi3 replicase preparation must contain ribosomal protein S1 for initiation to occur at low GTP. Mn2+ ions, which can substitute for host factor in Q/3 RNA replication in uitm, and which reduce the template specificity of QB replicase, also reduce the GTP requirement for initiation. But while Mn2+ ions produce this effect with all templates, host factor is specific for Q/3 RNA. When both host factor and Mn2+ are present, transcription of &B RNA occurs at a much lower GTP concentration. Thus, host factor and Mn2+ appear to reduce the GTP concentration requirement by different mechanisms.