Compositional bimodality and evolution of retroviral genomes.

The compositional distributions of genomes, genes (and their third codon positions) and long terminal repeats from retroviruses of warm-blooded vertebrates are characterized by a striking bimodality which is accompanied by a remarkable compositional homogeneity within each retroviral genome. A first, major class of retroviral genomes is GC-rich, whereas a second, minor class is GC-poor. Representative expressed viral genomes from the two classes integrate in GC-rich and GC-poor isochores, respectively, of host genomes. The first class comprises all oncoviruses (except B-types and some D-types), the second, lentiviruses, spumaviruses, as well as B-type and some D-type oncoviruses (e.g., mouse mammary tumor virus and simian retroviruses type D, respectively). The compositional bimodal distribution of retroviral genomes and the accompanying compositional homogeneity within each retroviral genome appear to be the result of the compositional evolution of retroviral genomes in their integrated form.