“Rule of Six”: How Does the Sendai Virus RNA Polymerase Keep Count?

The “rule of six” stipulates that the Paramyxovirus RNA polymerase efficiently replicates only viral genomes counting 6n + 0 nucleotides. Because the nucleocapsid proteins (N) interact with 6 nucleotides, an exact nucleotide-N match at the RNA 3′-OH end (3′-OH congruence) may be required for recognition of an active replication promoter. Alternatively, assuming that the six positions for the interaction of N with the nucleotides are not equivalent, the nucleotide position relative to N may be critical (N phase context). The replication abilities of various minireplicons, designed so that the 3′-OH congruence could be discriminated from the N phase context, were studied. The results strongly suggest that the application of the rule of six depends on the recognition of nucleotides positioned in the proper N phase context.