The Virome

THE VIROME

As you remember, the virome is the viral genome. There is more genomic diversity among viruses in nature than there are in archaea, bacteria, animals, and plants. More than 75,000 out of millions of different types of bacteria have had their genomes determined, although only 5000 viruses have been fully identified. We have also talked about the contents of the genome, with RNA or DNA making up the different viral particles. Most viruses have RNA genomes with plant viruses mostly being singlestranded RNA genomes and bacteriophages having mostly double-stranded DNA genomes.

Viral genomes can be circular or linear in nature. The genome is segmented in many cases. Each segment codes for a single protein, similar to genes in cellular DNA. Most are found in a single capsid; however, a few situations exist in which different infectious segments are found in different but interdependent viral particles. This is seen, for example, in the brome mosaic virus particles, which is a plant virus. It is also seen in other plant virus infections.

The viral genome is can be single-stranded or double-stranded DNA or RNA. Remember that most DNA comes double-stranded as a twisted helix that has two outer ladder poles and multiple rungs made up of base pairs. In single-stranded DNA and RNA, it is as though a ladder was cut vertically in half. The virus particles in Hepadnaviridae species have partially double-stranded and partially single-stranded nucleic acids. Single strands can be positive sense or negative sense, called plus strands and minus strands, respectively. Plus-strand RNA can be directly translated into proteins, while negative strand RNA is like a photo negative. It needs to be turned into a positive by RNAdependent RNA polymerase, an enzyme that makes a positive strand RNA, which can be read.

DNA in a virus particle can be double-stranded or single-stranded. A single strand can be positive or negative, with the possibility of what’s called ambisense nucleic acidsituation, which involves reading of the strand from both ends of the strand. This can happen with either ssDNA or ssRNA.