DNA Viruses

Some DNA viruses, such as the circoviruses and parvoviruses have single-stranded genomes, but the viruses we will concentrate on here all have double-stranded DNA as their genetic material. There are some 22 virus families and 90-odd genera containing hundreds of individual viruses which fall into class I of the Baltimore classification. These viruses are interesting because they range from some of the simplest viruses known to the most complex examples yet discovered. In order to simplify the situation, this article only describes five of these virus families.

The polyomaviruses some of the simplest viruses known, with circular genomes of just five kilobase pairs (kbp) containing five or six genes. Just because they are small does not mean that they are simple, and polyomavirus genomes contain a number of "advanced" features such as use of both strands of the DNA to encode proteins, overlapping reading frames and the production of alternative polypeptides via spliced mRNAs. Like most (but not all) DNA viruses, polyomaviruses replicate their genomes in the nucleus of the host cell. Since they encode only a few proteins, these viruses are the most heavily dependent on the host cell for replication. It is no accident that polyomavirus DNA is the most similar to eukaryotic chromosomal DNA, supercoiled in structure and associated with histones derived from the host cell. In the nucleus, the virus mini-chromosome is transcribed by host cell RNA polymerase II to produce early mRNAs. Transcription from the early region promoter is autoregulated by binding of a non-structural protein (i.e only found in virus-infected cells but not in the virus particle), known as T-antigen, to the regulatory region of the genome. The early gene promoter contains strong enhancer elements which cause it to be active in newly infected cells. As the concentration of T-antigen builds up, transcription of the early genes is repressed by binding of the protein to the origin region of the virus genome, preventing transcription from the early promoter and causing a switch to the late phase of infection. After DNA replication has occurred, transcription of the late genes results in synthesis of the virus structural proteins. This separation of replication into early and late phases (with different sets of genes expressed before and after genome replication) is characteristic of DNA viruses.

In contrast to polyomaviruses, adenoviruses are larger and more complex, with linear DNA genomes of 30-38 kbp, encoding 30-40 genes. As with polyomaviruses, replication of the adenovirus genome occurs in the nucleus of the host cell. Unlike polyomaviruses, the 40-odd virus genes are expressed from a number of separate promoters. Multiple protein products made from each gene by alternative splicing of the mRNA transcripts.

Herpesviruses are even more complex than adenoviruses, with genomes of 105-235 kbp, containing 80-100 genes. Once again, replication occurs in the nucleus, with each gene being transcribed from its own individual promoter.

Poxviruses represent a further increase in complexity, with genome of 130-300 kbp, containing around 250 genes. However, they have also learned a new trick, as poxvirus genome replication occurs in the cytoplasm rather than the nucleus of the host cell. Poxviruses are able to achieve this because they have sufficient genetic capacity to encode all the factors needed to replace those responsible for host DNA replication in the nucleus. For the poxvirus, the advantages are that they are able to achieve much greater control of replication, and forming and releasing virus particles in the cytoplasm rather than the nucleus is presumably simpler.

The largest and most complex DNA viruses known are larger both in terms of particle size and genome size than the smallest bacteria, so upsetting the statement sometimes made that "viruses are smaller than bacteria". Most often that is true, but not in the case of the biggest viruses such as Mimivirus.

In summary, class I DNA viruses range from some of the simplest to some of the most complex viruses known. Their replication is typically divided into early and late phases with different sets of genes expressed before and after genome replication. In addition to many tailed bacteriophages such as lambda, there are also some important pathogens in this class, including herpesviruses and smallpox.