Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 139

The question “does genome size matter” is difficult to answer considering the three orders of magnitude in genome length that separate the largest and the smallest viral genomes. The two largest viral genomes known are those of Pandoravirus salinus (2.4 million bases of dsDNA) and Pandoravirus dulcis (1.9 million bases of dsDNA), encoding 2,541 and 1,487 open reading frames, respectively. The largest RNA virus genomes are far behind (Box 3.4). At the other end are anelloviruses, with a 1,759-base ssDNA genome encoding two proteins (Fig. 3.3B), and viroids, circular, single-stranded RNA molecules of 246 to 401 nucleotides that encode no protein (Volume II, Chapter 13). Anelloviruses include agriculturally important pathogens of chickens and pigs and torque teno (TT) virus, which infects >90% of humans with no known consequence. Viroids cause economically important diseases of crop plants.

All viruses with genome sizes spanning the range from the biggest to the smallest are successful as they continue to reproduce and spread within their hosts. Despite detailed analyses, there is no evidence that one size is more advantageous than another. All viral genomes have evolved under relentless selection, so extremes of size must provide particular advantages. One feature distinguishing large genomes from smaller ones is the presence of many genes that encode proteins for viral genome replication, nucleic acid metabolism, and countering host defense systems. When mimiviruses were first discovered, the surprise was that their genomes encoded components of the protein synthesis system, such as tRNAs and aminoacyl-tRNA synthetases. Tupanviruses, isolated from soda lakes in Brazil and deep ocean sediments, encode all 20 aminoacyl-tRNA synthetases, 70 tRNAs, multiple translation proteins, and more. Only the ribosome is lacking. Why would large viral genomes carry these genes when they are available in their cellular hosts? Perhaps by producing a large part of the translational machinery, viral mRNAs can be more efficiently translated. This explanation is consistent with the finding that the codon and amino acid usage of tupanvirus is different from that of the amoeba that it infects.