Abstract
Essential genes are thought to be critical for the survival of the organisms under certain circumstances, and the natural selection acting on essential genes is expected to be stricter than on nonessential ones. Up to now, essential genes have been identified in approximately thirty bacterial organisms by experimental methods. In this paper, we performed a comprehensive comparison between the essential and nonessential genes in the genomes of 23 bacterial species based on the Ka/Ks ratio, and found that essential genes are more evolutionarily conserved than nonessential genes in most of the bacteria examined. Furthermore, we also analyzed the conservation by functional clusters with the clusters of orthologous groups (COGs), and found that the essential genes in the functional categories of G (Carbohydrate transport and metabolism), H (Coenzyme transport and metabolism), I (Transcription), J (Translation, ribosomal structure and biogenesis), K (Lipid transport and metabolism) and L (Replication, recombination and repair) tend to be more evolutionarily conserved than the corresponding nonessential genes in bacteria. The results suggest that the essential genes in these subcategories are subject to stronger selective pressure than the nonessential genes, and therefore, provide more insights of the evolutionary conservation for the essential and nonessential genes in complex biological processes.
Highlights
It is well known that the rates of evolution have significant variations among protein-coding genes
We examined evolutionary conservation based on the clusters of orthologous groups of proteins (COGs), and found that the essential genes in the COG functional categories G, H, I, J, K and L tend to have a lower rate of evolution compared with the corresponding nonessential genes
In order to estimate the difference of evolutionary conservation between the essential and nonessential genes, we constructed a data set containing the essential and nonessential genes, which were determined in genome-wide screen
Summary
It is well known that the rates of evolution have significant variations among protein-coding genes. The key principle for the identification of the essential genes is that the function absence of normal genes results in lethality or infertility in some special conditions[2]. Given these results, it is likely that the essential genes have a greater level of purifying selection pressure during the natural evolution. Along with the availability of the essential genes identified by experiment from the DEG database, the previous finding, that the essential genes are evolutionarily conserved than the nonessential genes, was confirmed with 23 genomes of bacterial organisms. The results suggest the difference between the essential and nonessential genes in terms of evolutionary rates among various functional categories, and provide further insights into the evolutionary pressures acting on the essential and nonessential genes
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