CAN LARGE dsDNA-CONTAINING VIRUSES PROVIDE INFORMATION ABOUT THEMINIMAL GENOME SIZE REQUIRED TO SUPPORT LIFE?

The genomes of a few viruses, such as Bacillus megaterium phage G (670 kb) and the chlorella viruses (330 to 380 kb), are larger than the predicted minimal genome size required to support life (ca. 320 kb). A comparison of the 256 proteins predicted to be required for life with the putative 376 proteins encoded by chlorella virus PBCV-1, as well as those encoded by other large viruses, indicates that viruses lack many of these “essential” genes. Consequently, it is unlikely that viruses will aid in determining the minimal number and types of genes required for life. However, viruses may provide information on the minimal genome size required for life because the average size of genes from some viruses is smaller than those from free-living organisms. This smaller gene size is the result of three characteristics of virus genes: (1) virus genes usually have little intragenic space between them or, in some cases, genes overlap; (2) some virus-encoded enzymes are smaller than their counterparts from free-living organisms; and (3) introns occur rarely, if at all, in some viruses.