Ribosomal protein S1 functions as a termination factor in RNA synthesis by Qβ phage replicase.

S1 is the largest ribosomal protein, and is vitally important for the cell. S1 is also a subunit of Qβ replicase, the RNA-directed RNA polymerase of bacteriophage Qβ. In both protein and RNA syntheses, S1 is commonly believed to bind to a template RNA at the initiation step, and not to be involved in later events.