Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 271

As polymerases synthesize nucleic acid in a 5′ to 3′ direction, the nucleotidyl transfer reaction is initiated at the 3′ end of the template strand. The requirement for a primer for initiation of nucleic acid synthesis varies among the different classes of polymerases. Most DNA polymerases are primer-dependent enzymes, while DNA-dependent RNA polymerases initiate RNA synthesis de novo. Some RdRPs (e.g., those of flaviviruses and rhabdoviruses) can also initiate RNA synthesis de novo, while others require a primer (Fig. 6.7). In these cases, RNA polymerization is initiated by a protein-linked primer (picornaviruses) or an oligonucleotide cleaved from the 5′ end of cellular pre-mRNA (influenza viruses).