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In this article we will discuss about the replication of adenoviruses.
Adenovirus replicates inside the nucleus (Fig. 17.9). By the DNA replication process, the adenovirus multiplication cycle is separated into two phases i.e., an early phase and a late phase.
In both phases a primary transcript is formed which is spliced to produce monocistronic mRNA which is compatible with the host’s ribosome. This allows the translation of mRNAs. Mainly non-structural and regulatory proteins are expressed by the early genes.
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These proteins function:
(i) To alter the expression of host proteins that are necessary for DNA synthesis;
(ii) To activate other virus genes (such as the virus-encoded DNA polymerase), and
(iii) To avoid premature death of the infected cell by the host-immune defenses (blockage of apoptosis, interferon activity, and translocation and expression of MHC Class I).
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E1 A is the first mRNA/protein to be made about 1 h after infection. This protein is a transacting transcriptional regulatory factor whose mode of action is not known, but it is necessary for activation of transcription of early genes. The second protein synthesized is E1 B. E1 A + E1 B together (and independently of other virus proteins) is capable of transforming the primary cells in vitro (Fig. 17.9).
There are at least three virus-encoded proteins that are involved in DNA replication: terminal protein (TP) (acts as a primer for initiation of synthesis), a DNA-binding protein (DBP), and DNA Pol (140 kD DNA-dependent polymerase).
In addition, many cellular proteins in the nucleus also participate in replication of the genome [e.g. nuclear factors (NFI, NFII), topoisomerase I]. The adenovirus genome has inverted terminal repeats (ITRs) of about 100 bp.
Within the ITRs, cis-acting DNA sequences are located which define the origin of DNA replication (ori). A terminal protein (TP) is covalently attached to each 5′ end which acts an additional cis-acting component of ori gene.
The terminal 18 bp of genome directs the limited initiation of replication just involving the three viral proteins: pre-terminal protein (pTP), DNA polymerase (Pol) and DNA binding protein (DBF).
However, two cellular transcription factors, NFI and NFIII are required for efficient levels of replication. In contrast adenovirus-4 replicates efficiently without NFI and NFIII. Further a cellular factor, a topoisomerase, is required for complete elongation. DNA replication of adenovirus involves several steps.
First, the viral genome is coated with DNA binding protein and converted into a virus DNA- cell histone complex. This protein reacts co-operatively with the cellular transcription factor, NFI which binds to a recognition site within the origin of replication.
NFIII also binds at a specific recognition site between nucleotides 39 and 48. Protein-protein interactions between NFI and DNA Pol, and pTP and NFIII help recruit the pTP-DNA Pol heterodimer into the pre-initiation complex.
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When DNA replication is started, the pattern of transcription changes from the early to the late genes. Since there is cis-acting control of this switch, only newly replicated DNA is used for late gene transcription; but the mechanism controlling this is not understood.
Late phase transcription is driven mainly by the major late promoter. Transcription from this promoter is complex which involves multiple polyadenylation signals and a usage of RNA splicing. Five gene clusters (L1-L5) can be defined.
Gene expression at late phase mainly results in synthesis of virion proteins. Viral assembly begins in the cytoplasm with formation of hexon and penton capsomers from individual monomer virions but complete packaging is accomplished in the nucleus.
Empty, immature capsids are assembled from these protomers in the nucleus, where the core is formed from genomic DNA + associated core proteins. Thereafter, newly synthesized progeny virions are released from the host cells.
