Some viruses also encode special DNA polymerases, such as Hepatitis B virus DNA polymerase. In contrast to bacterial RNA polymerases, T7 polymerase is not inhibited by the antibiotic rifampicin. Two replicative polymerases synthesize DNA in opposite orientations. Sigma factors are specialized bacterial proteins that bind to RNA polymerases and orchestrate transcription initiation. Protein synthesis within chloroplasts relies on two RNA polymerases. The three eukaryotic polymerases share four other common subunits among themselves. In eukaryotes, however, nucleosomes act as major barriers to transcribing polymerases during transcription elongation. Virus RNA polymerases use VPg as primer. Examples of protein complexes include the proteasome for molecular degradation and most RNA polymerases. Both of these polymerases are important for ribosome biogenesis. Eukaryotes have three nuclear RNA polymerases, each with distinct roles and properties. The two polymerases are bound to the helicase heximer. Its catalytic domain is homologous to that of other polymerases. The other multi-subunit RNAP lineage formed all of the modern cellular RNA polymerases. New strands are created by enzymes called DNA polymerases.