Proteolytic processing of the Sindbis virus non‐structural polyproteins (P123 and P1234) and synthesis of minus‐ and plus‐strand RNAs are highly regulated during virus infection. Although their precise roles have not been defined, these polyproteins, processing intermediates or mature cleavage products (nsP1‐4) are believed to be essential components of viral replication and transcription complexes. In this study, we have shown that nsP4 can function as the polymerase for both minus‐ and plus‐strand RNA synthesis. Mutations inactivating the nsP2 proteinase, resulting in uncleaved P123, led to enhanced accumulation of minus‐strand RNAs and reduced accumulation of genomic and subgenomic plus‐strand RNAs. In contrast, no RNA synthesis was observed with a mutation which increased the efficiency of P123 processing. Inclusion of this mutation in a P123 polyprotein with cleavage sites 1/2 and 2/3 blocked allowed synthesis of both minus‐ and plus‐strand RNAs. We conclude that nsP4 and uncleaved P123 normally function as the minus‐strand replication complex, and propose that processing of P123 switches the template preference of the complex to minus‐strands, resulting in efficient synthesis of plus‐strand genomic and subgenomic RNAs and shut‐off of minus‐strand RNA synthesis.