Authors: F Takeuchi, S Watanabe, T Baba, K Hiramatsu
Abstract title: Speciation of Staphylococci and the oriC environ
Introduction. We compared the genomes of Staphylococcus aureus, S.
epidermidis and S. haemolyticus in order to study their evolution.
Methods. All of the open reading frames (orfs) from the
staphylococcal and related species were classified into "homologs"
based on the nucleotide sequence similarity. The orfs with a quality
ratio by alignment >=70% were grouped together to form a homolog. In
order to determine which homologs were common to staphylococcal
species and which homologs were unique, we classified the homologs by
their phylogenetic profile, i.e., the pattern of appearance in the
three staphylococcal species.
Results. The distribution of the homologs common to staphylococci on
each chromosome distinguished the regions by horizontal transfer from
those vertically transmitted. The orfs common to the three
staphylococcal species were conserved in their sequences and their
order on chromosomes, encoded genes fundamental for bacteria, and
constituted the "genomic backbone" of each species. On the other
hand, we found chromosomal regions with few common orfs, which
corresponded to transposons, integrated plasmids, prophages, or other
genomic islands. We also found that the region in the downstream of
oriC (origin of DNA replication) from 30 kbp down to 0.4--0.6 Mbp
were specific to each species, and designated as "oriC environ." The
oriC environ included in its right end the staphylococcal cassette
chromosome (SCC) which is a mobile genetic element that brings
antibiotic resistance to staphylococcal strains.
Discussion. We also found that mutant strains of S. haemolyticus had
large deletions of up to 427 kbp within the oriC environ, which
altered their biochemical properties. Thus the oriC environ is not
essential for bacterial viability but characterizes the phenotypes of
each species. We think this region was formed by accumulating foreign
genes, which suggests a novel mechanism for speciation. (J.
Bacteriol. (2005) 187, 7292--7308)
Last modified: Wed Sep 13 15:20:16 2006