Exciting developments in virus research
The “virus group” at the Institute of Microbiology were among the first to demonstrate that there are a fantastic number of viruses in the environment. Now they are showing some interesting things about virus complexity.
Not only has it been demonstrated that there are an incredible number of viruses out there (10 000 000 per ml of seawater), but recent work has identified viruses that contain a genome larger than what was believed to be the minimal size for independent life.
Virus particles have come to be recognised as being significant players in the eco-system. For example, it seems increasingly likely that they play some kind of role in controlling algal blooms. This has important implications for possibly finding ways of limiting the damage caused by some of these blooms. In addition, it is also possible that these tiny particles have an impact on global climate. Some of the algae they infect are responsible for producing compounds (precursors of dimethysulphide (DMS)) that directly affect cloud and rain cycles.
Genomes and life
The size of the genome (the amount of genetic material) of living cells varies depending on the cell, and type of organism. The genome contains all the “recipes” for all the metabolic processes within a cell, and for the making of all the “players” in these processes. Thus, even the smallest, simplest cell has to have a genome of a certain size in order to contain the minimum amount of information required for the simplest life.
A typical virus genome contains only the code for a few proteins, such as its particular protein coat, and a few transcription factors. It “borrows” or uses most of the genomic information it needs to replicate itself from the host cell’s genome. Thus a virus genome is typically very small. Until recently, it was believed to be well under the minimum size to have enough information for life.
However, Ruth-Anne Sandaa, et al, at the Department of Microbiology have recently identified two viruses whose genome is larger than this minimum value. What does this mean? Are these genome-rich viruses on the threshold between living and non-living? What does this mean about the way they operate and infect their hosts? Or, is the size of their genome simply due to multiple copies of a few genes?
What is life?
Currently viruses occupy a kind of grey zone between living and non-living. In the environment viruses are inert packages of genomic material. In some viruses this genomic material consists of RNA, in others of DNA, and in still others it consists of double stranded DNA, such as we have ourselves in the nuclei of our body cells. Within a host cell, however, this inert genetic material often takes over some or all of the cell’s machinery, and uses it to reproduce new virus particles.
If the definition of life or being alive is based on being able to reproduce one’s self, then can viruses be considered living? What about these new viruses with the large genomes - what are they doing?
What is next?
The “virus group” here in Bergen is primarily interested in the ecology of viruses - how do viruses, and their relationship with their hosts, fit into the whole ecological picture? The group is going to continue to study the genes and population dynamics of these new viruses, in the hopes of getting a clearer picture of the ecology of the whole marine eco-system. They plan to collaborate with molecular biologists for help with the sequencing work.
Sandaa is particularly conversant with a new technique called pulse field gel electrophoresis (PFGE). Two years ago she travelled to Maryland to learn more about the technique from Russell Hill, a researcher whose group was one of the first to use this technique to study the viral communities in natural marine samples. Sandaa uses the technique to measure the viral diversity in samples. She feels that it is unfortunate that the technique only differentiates viral particles by size, and not by type. However, with time and experience, she says that you come to see the same particular sizes in results from different samples, and it may be that these particular sizes may represent particular virus types.
Sandaa says that the group would now like to sequence the genomes of the two new algal viruses. The sequence information can be compared with the growing wealth of other genetic sequence information. Such comparisons would reveal whether the virus genome contains the code for any known genes. The presence of known genes would make it possible to speculate about potential functions or activities of the viruses. The sequence information can also be used in comparison with sequence information from other viruses to classify and compare the new viruses with other, known viruses.
Controlling viral blooms
One of the questions the “virus group” will be keeping in mind relates to the potentially controlling role that viruses have in algal blooms. Currently there are two theories as to how viruses may do this. One is based on population studies. These studies show that a peak in algal cell population (a bloom), is closely followed by a peak in the virus population. It is as if the high density of the host cells (the bloom) permits greater contact between host cell and virus particle. The viruses can then “reproduce” in larger numbers, ultimately reducing the host cell population and ending the bloom.
In the second theory, the viruses are thought to “control” the bloom by preventing it from happening in the first place. In this theory, the insertion of the virus particle into the host cell, somehow prevents that cell from undergoing uncontrolled reproduction, such as occurs during a bloom.
Further understanding of the biology of the two new viruses may help the group to elucidate more information about the potential controlling mechanisms that viruses have over blooms. One of the new viruses infects an algal species that undergoes periodic blooms (and the virus population has been observed to undergo a parallel bloom shortly afterwards), while the other infects a species that has not yet been observed to undergo a bloom.