Relationship of Genomic G+C Content between Phages/Plasmids and Their Hosts

Aims: We compared genomic G+C content between bacteriophages/plasmids and their host genomes of 46 species to reveal their relation. To examine the large deviation in the G+C content between bacteriophages and their host genomes, the ancestral bacteriophage which infected early was estimated using homologous genes of bacteriophages based on G+C content at the third codon positions.

Place and Duration of Study: Department of Clinical Laboratory Science, Graduate Course of Medical Science and Technology, School of Health Sciences, Kanazawa University, Japan.

Methodology: The bacteria employed in this study consist of 6 species from Archaea and 40 species from Eubacteria. Orthologs were identified by the two-directional best hit approach of homology search. A phylogenetic tree was obtained by multiple sequence alignment of homologous genes. The ancestral bacteriophage which infected early was estimated based on G+C content at the third codon positions. We assumed that the two bacteriophages have evolved from a common ancestor, and their identical codons were thought to represent their ancestor type.

Results: The relationship of G+C content between bacteriophages/plasmids and host genomes was almost linear. Three bacteriophages were largely deviated from the linear relation. A phylogenetic tree obtained using the orthologs of Mycobacterium smegmatis indicated which bacteriophage branched early. Assuming that the G+C content of identical codons represents their common ancestor, the ancestor was estimated that it had similar G+C content with its host.