SUMMARYChanges in DNA polymerase activities were studied in pupae of the silkworm Bombyx mori upon infection with nuclear polyhedrosis virus (BmNPV). Two effects were observed in male and female pupae after inoculation of BmNPV: a marked increase in cellular a-polymerase activity and the induction of a new DNA polymerase (BmNPV-polymerase). Activities of both DNA polymerases, ct and BmNPV, in the infected pupae increased in parallel, reached a maximum at 72 h post-inoculation and decreased subsequently. The virus infection did not affect the activity of y-polymerase in the pupae, An increased activity of fl-polymerase in the pupae was detected at the late stage of the infection cycle prior to pupal death. The possible roles of the DNA polymerases in BmNPV replication are discussed.Nuclear polyhedrosis viruses (NPV) of insect species contain a double-stranded, covalently closed circular DNA of approximately 120 to 130 kilobase pairs (Bud & Kelly, 1977;Miller & Dawes, 1979). The mechanism of NPV DNA replication in infected cells is unknown as yet, and information about enzymes that are involved in NPV replication is limited (for review, see Kelly, 1982). It was observed that inoculation of NPV causes a considerable increase in overall DNA polymerase activity in silkworm pupae (Onodera et al., 1968). Stimulation of DNA polymerase by baculoviruses has been demonstrated also in Spodopterafrugiperda cells (Kelly, 1981). Whether cellular or virus-specific DNA polymerases were activated during virus infection remains unclear, but the presence of virus-encoded DNA polymerase in cells infected with NPV has been suggested recently (Miller et al., 1981;Wang & Kelly, 1983).In the present paper, we describe analysis of the activities of different DNA polymerases in pupae of the silkworm Bombyx mori after infection with nuclear polyhedrosis virus (BmNPV). It was shown that infection of silkworm pupae by this virus results in the induction of a virusspecific DNA polymerase (BmNPV-polymerase) as well as in a marked increase in the activity of host cell ct-polymerase. Purification of BmNPV polymerase from the infected pupae and properties of this enzyme will be described elsewhere (V. S. Mikhailov et al., unpublished).Preparation of BmNPV and infection of the silkworm pupae were performed as described earlier (Onodera et al., 1965). BmNPV was injected into the pupae on the third day of the pupal period. Control uninfected pupae were injected with the same buffer as the infected pupae but without BmNPV. Infected and uninfected pupae were incubated at 25 °C, sampled daily for biochemical analysis and stored in liquid nitrogen. The mortality of infected pupae was observed 6 days post-inoculation of BmNPV. Extracts of the silkworm pupae for ultracentrifugation in glycerol gradients were prepared as described previously for loach cells (Mikhailov & Gulyamov, 1983). Each extract was prepared from five pupae. An extract from silkworm embryos stored in diapause was obtained by the same method. Aliquots of the extracts (1.5 to 3 mg protei...
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