Roles of the host polypeptides in Q beta RNA replication. Host factor and ribosomal protein S1 allow initiation at reduced GTP concentration.

Initiation of transcription of favored templates by Q beta 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 Q beta RNA, the presence of the host factor substantially reduces the GTP concentration requirement. In addition, the Q beta replicase preparation must contain ribosomal protein S1 for initiation to occur at low GTP. Mn2+ ions, which can substitute for host factor in Q beta RNA replication in vitro, and which reduce the template specificity of Q beta replicase, also reduce the GTP requirement for initiation. But while Mn2+ ions produce this effect with all templates, host factor is specific for Q beta RNA. When both host factor and Mn2+ are present, transcription of Q beta RNA occurs at a much lower GTP concentration. Thus, host factor and Mn2+ appear to reduce the GTP concentration requirement by different mechanisms.