Evolution of genetic information flow from the viewpoint of protein sequence similarity.
As a course of inquiry into the evolution of genetic information flow, similarity relations of amino acid sequences between the proteins involved in translation, transcription and replication are investigated. The sequence data of these proteins are mostly accumulated from Escherichia coli, and the present investigation is carried out mainly on this organism by the FASTP program. This result reveals an interesting similarity linkage extending from ribosomal proteins to the proteins participating in translational elongation process and to the proteins in transcription and replication. Although the ribosomal proteins are of relatively short polypeptide chains, our systematic comparison between these proteins finds many similarity relations, being more than 100 in terms of "overlap", reducing them to about 14 elementary ribosomal proteins from which other ribosomal proteins would have diverged. Moreover, the proteins involved in translation, transcription and replication contain the regions similar to the elementary ribosomal proteins. In particular, some initiation and elongation factors in translation process are assigned to be similar to the elementary ribosomal proteins almost over the whole regions. To such an elongation factor Tu, the alpha and sigma 70 subunits of RNA polymerase and primase also show similarity in the wider regions than the individual ribosomal proteins, and they are shown to be fundamental for the similarity linkage extending to the other polypeptide chains involved in transcription and replication processes, although the latter polypeptide chains contain regions not similar to any ribosomal protein. This divergence pattern of similarity relations strongly suggests that the proteins involved in the contemporary genetic information flow DNA-->RNA-->protein have evolved from some elementary ribosomal proteins, first by gene fusion, in a primitive organism of the RNA-protein world, and then by the addition of the mechanism of domain shuffling from other genes in the DNA-RNA-protein world.