An Alternative to the RNA World - PubMed (original) (raw)
An Alternative to the RNA World
Charles W Carter Jr. Nat Hist. 2016 Dec.
No abstract available
Figures
Fig. 1
Protein synthesis by ribosomes. Bases in mRNA have different shapes that fit together with only the right anticodon at one end of tRNA. Readout proceeds left to right, from 5′ to 3′, creating a new peptidyl-tRNA one amino acid longer than the previous one. As a result, the bond connecting the growing peptide to the tRNA on the left is broken and that tRNA is released (red arrows). The new peptidyl-tRNA advances to the left, exposing the next codon and creating an opening for the next aminoacyl-tRNA on the right. The larger 50S subunit of the ribosome(salmon) is responsible for making peptide bonds between the growing peptide and the incoming aminoacyl-tRNA. The 30S subunit (blue) decodes mRNA by enforcing correct codon-anticodon pairing.
Fig. 2
Inferring protein family trees from molecular anatomies. Structures of three class I and class II aaRS have been rotated into a common orientation using their atomic coordinates and colored with different colors. α-Helical secondary structures are drawn as cylinders; extended β-structures as ribbons with arrows indicating their direction. Larger, more differentiated structures are drawn as noodles, surrounded by their surfaces. Differences in the recent structures at the bottom are highlighted by modules of one color that are absent in other structures. Such differences can be quantified and used to construct the genealogies in the center. Modules that are most similar in all three are colored dark blue and are inferred to be present in the common ancestor. Circles represent essentially modern aaRS. The three structures in each aaRS class are labeled with their three-letter abbreviations. There is consensus that they were present in the last universal common ancestor (LUCA) of all living organisms. Novel results described here are the construction, expression, and experimental testing of ancestral forms called urzymes and protozymes, which are found, essentially without variation in all contemporary species and which retain substantial fractions—60% and 40% respectively—of the catalytic activity of the contemporary enzymes. The similarity between the class I and II genealogies is evidence that the two families evolved coordinately.
Fig. 3
Sense/antisense coding of ancestral class I and II protozymes. One double-stranded gene constructed by the computer program, Rosetta, produces two different, equally functional protozymes from the instructions in opposite strands. Each strand is thus a gene as well as a template. For obvious reasons, it is tempting to follow Genesis and call the two strands Αδαμ and Εωε, using Greek names to distinguish these molecular ancestors from our mythical human ancestors. Although there is only a single unique set of instructions, that unique information has two distinct and functional interpretations, depending on which strand is read, as in the illustration on the right.
Fig. 3
Sense/antisense coding of ancestral class I and II protozymes. One double-stranded gene constructed by the computer program, Rosetta, produces two different, equally functional protozymes from the instructions in opposite strands. Each strand is thus a gene as well as a template. For obvious reasons, it is tempting to follow Genesis and call the two strands Αδαμ and Εωε, using Greek names to distinguish these molecular ancestors from our mythical human ancestors. Although there is only a single unique set of instructions, that unique information has two distinct and functional interpretations, depending on which strand is read, as in the illustration on the right.
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