EVOLUTION OF TRANSCRIPTIONAL REGULATION SYSTEM THROUGH PROMISCUOUS COUPLING OF REGULATORY PROTEINS WITH OPERONS - SUGGESTION FROM PROTEIN-SEQUENCE SIMILARITIES IN ESCHERICHIA-COLI

As an advanced molecular study of the problems of the evolution of org anisms, the transcriptional regulation system is studied by investigat ing the amino acid sequence similarities between the proteins in the r egulation system of Escherichia coli in which the data of sequenced pr oteins as well as of regulator-regulon relationships are accumulated. The similarities between the proteins are calculated by the FASTA algo rithm and their homology is also evaluated in terms of statistical sig nificance with the use of the RDF2 program. This investigation reveals that the similarity between the regulatory protein and the regulated protein is hardly found, but many similarities are found between regul atory proteins and between regulated proteins. These similarity relati ons are compared with the regulator-regulon relationships ascertained experimentally. From this comparison, it is found that similar regulat ory proteins rarely regulate the transcription of similar protein gene s. As most of the highly similar proteins are considered to have diver ged from a common ancestral protein, this finding strongly suggests th e possibility that descendant regulatory proteins have been promiscuou sly coupled with descendant operons, independently of their ancestral regulator-regulon relationship, and that some of the couplings have be en fixed by selection to form the present system of transcriptional re gulation. The compatibility of such promiscuous coupling with regulato ry organization is illustrated in the carbohydrate transport systems a nd the succeeding metabolic pathways, whose organization is comprehens ive in sending nutritious substances to the central path of glycolysis under different environmental conditions. The benefit of flexibility in regulator-regulon relationships in evolutionary processes is also d iscussed in connection with the punctuational divergence of species in macroevolution and the cell differentiation in multicellular organism s. (C) 1996 Academic Press Limited