Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 207

Many types of virus particles contain enzymes necessary for synthesis of viral nucleic acids. These enzymes generally catalyze reactions unique to virus-infected cells, such as synthesis of viral mRNAs from an RNA template or of viral DNA from an RNA template (retroviral reverse transcriptases). However, virions of vaccinia virus contain a DNA-dependent RNA polymerase, analogous to cellular RNA polymerases, as well as several enzymes that modify viral RNA transcripts (Table 4.3). This complement of enzymes is necessary because transcription of the viral double-stranded DNA genome takes place in the cytoplasm of infected cells, whereas cellular DNA-dependent RNA polymerases and the RNA-processing machinery are restricted to the nucleus. Other types of enzymes found in virus particles include integrase, cap-dependent endonuclease, and proteases. The proteases sever covalent connections within polyproteins or precursor proteins from which some virus particles assemble, a reaction that is necessary for the production of infectious particles (Chapter 13).