Large' DNA Genomes

There are a number of virus groups which have double-stranded DNA genomes of considerable size & complexity. In many respects, these viruses are genetically very similar to the host cells which they infect. Two examples of such viruses are the adenovirus & herpesvirus families:

Herpesvirus genomes:

The herpesviruses are a large family containing more than 100 different members, at least one for most animal species which have been examined to date, including seven human herpesviruses.

Herpesviruses have very large genomes composed of up to 230kbp linear, double-stranded DNA. The different members of the family are widely separated in terms of genomic sequence & proteins, but all are similar in terms of structure & genome organization.

Some herpesvirus genomes consist of two covalently joined sections, a unique long (UL) & a unique short (US) region, each bounded by inverted repeats. The repeats allow structural rearrangements of the unique regions & therefore, these genomes exist as a mixture of four isomers, all of which are functionally equivalent:

Herpesvirus genomes also contain multiple repeated sequences & depending on the number of these, the genome size of various isolates of a particular virus can vary by up to 10kbp.

Adenovirus genomes:

The genomes of adenoviruses consist of linear, double-stranded DNA of 30-38kbp. These viruses contain 30-40 genes. The terminal sequences of each DNA strand are inverted repeats of 100-140bp & therefore, the denatured single strands can form 'panhandle' structures. These structures are important in DNA replication.

Although adenovirus genomes are considerably smaller than those of herpesviruses, the expression of the genetic information is rather more complex. Clusters of genes are expressed from a limited number of shared promoters. Multiply-spliced mRNAs & alternative splicing patterns are used to express a variety of polypeptides from each promoter. In contrast, herpesvirus genes each tend to be expressed from their own promoter - resulting in a much larger genome.

Segmented & Multipartite Virus Genomes

• Segmented virus genomes are those which are divided into two or more physically separate molecules of nucleic acid, all of which are then packaged into a single virus particle.
• Multipartite genomes are those which are segmented & where each genome segment is packaged into a separate virus particle. These discrete particles are structurally similar & may contain the same component proteins, but often differ in size depending on the length of the genome segment packaged.

There are many examples of segmented virus genomes, including many human, animal & plant pathogens such as orthomyxoviruses, reoviruses & bunyaviruses. There are rather fewer examples of multipartite viruses, all of which infect plants. These include:

• bipartite viruses (which have two genome segments/virus particles)
• tripartite viruses (three genome segments/virus particles)

Separating the genome segments into different particles removes the requirement for accurate sorting, but introduces a new problem in that all of the discrete virus particles must be taken up by a single host cell to establish a productive infection. This is perhaps the reason why multipartite viruses are only found in plants. Many of the sources of infection by plant viruses, such as inoculation by sap-sucking insects or after physical damage to tissues, results in a large input of infectious virus particles, providing the opportunity for infection of an initial cell by more than one particle.

SUMMARY:

1. There is more genetic diversity among viruses than in all the rest of the Animal, Plant & Bacterial kingdoms, all of whose genomes consist of d/s DNA.
2. The expression of virus genetic information is dependent on the structure of the genome of the particular virus concerned, but in every case, the genome must be recognized & expressed using the mechanisms of the host cell.

Дата добавления: 2014-01-11