RNAdependent RNA polymerase An

enzyme unique to viruses, encoded by the viral genome, which is specific for its own viral RNA and does not function for other virus species. It brings about transcription of viral RNA to complementary RNA and this is in turn transcribed back to form more viral RNA. In viruses where the RNA is a positive strand and can act as mRNA, translation commences immediately on infection and RNA-dependent RNA polymerase molecules are produced. These attach to the viral RNA and pass along from the 3' end producing a complementary strand which is thus synthesized from 5' to 3'. This complex of one positive strand with about six or seven complementary strands growing on it is called the 'replicative intermediate'. The presence of positive strands and complementary strands results in the formation of some double-helical strands, which are known as the 'replicative form'. This is an irrelevant end-product and much of it may be formed during the process of extraction from the cells, particularly if phenol is used. In those viruses where the RNA is a negative strand, the viral ribonucleoprotein has RNA-dependent RNA polymerase activity which means that mRNAs are made as well as complementary RNA to act as template for new virion negative-strand RNA synthesis. RNA-dependent RNA polymerases which catalyze the transcription of mRNA from the virion negative strand are also termed 'RNA transcriptases'; those which catalyze the formation of new virion RNA of whatever polarity are called 'RNA replicases'. Synonyms: RNA replicase; RNA syn-thetase; RNA transcriptase.

RNA-DNA viruses A name sometimes used for the family Retroviridae.

RNA ligase An enzyme isolated from bac-teriophage T4-infected Escherichia coli which adds single residues (pNp) to the 3' termini of RNA chains. Used experimentally to label RNA prior to sequencing by the gel method.

RNA polymerase An enzyme which catalyzes either the formation of RNA from a DNA template, in which case it is DNA-dependent RNA polymerase, or the formation of RNA from an RNA template, in which case it is RNA-dependent RNA polymerase.

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