Three into One Makes Three

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Science  31 Mar 2006:
Vol. 311, Issue 5769, pp. 1836
DOI: 10.1126/science.311.5769.1836a

Molecular explorations into the origins of the three major cellular domains—Archaea, Bacteria, and Eukarya—have generated warring interpretations of their differences and similarities. For instance, the components of the translational machinery (ribosomal RNAs and proteins) serve as a distinctive identifier for each domain, whereas some of the enzymes involved in DNA replication (as well as recombination and repair) are shared (in the sense of being homologs) between two domains, though not consistently the same two.

Forterre discusses a scenario in which the initiating events for converting a primordial common ancestor (a cell containing an RNA genome) into the modern-day triumvirate were infection and transformation (via a plasmid-like intermediate stage) by three DNA viruses. The substitution of DNA for RNA as the cellular genetic repository is postulated to have reduced the rate of evolution of proteins and to have established a barrier to subsequent takeovers. It is not clear whether the long-standing problems that this proposal addresses will simply be replaced by new ones, but the reminder that viral lineages are also a part of the early landscape is welcome. Indeed, structural analyses have placed viruses with an enormous range of host specificity (bacteriophage PRD1, Paramecium bursaria Chlorella algal virus, and mammalian adenovirus) in the same family on the basis of their major capsid protein (MCP) architectures, as revealed most recently by Khayat et al. for the Sulfolobus turreted icosahedral virus (STIV) and by Laurinmäki et al. for bacteriophage Bam35. — GJC

Proc. Natl. Acad. Sci. U.S.A. 103, 3669 (2006); 102, 18944 (2005); Structure 13, 1819 (2005).

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