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2015 ; 16
(ä): 817
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Transmission of the PabI family of restriction DNA glycosylase genes: mobility
and long-term inheritance
#MMPMID26481899
Kojima KK
; Kobayashi I
BMC Genomics
2015[Oct]; 16
(ä): 817
PMID26481899
show ga
BACKGROUND: R.PabI is an exceptional restriction enzyme that functions as a DNA
glycosylase. The enzyme excises an unmethylated base from its recognition
sequence to generate apurinic/apyrimidinic (AP) sites, and also displays AP lyase
activity, cleaving the DNA backbone at the AP site to generate the 3'-phospho
alpha, beta-unsaturated aldehyde end in addition to the 5'-phosphate end. The
resulting ends are difficult to religate with DNA ligase. The enzyme was
originally isolated in Pyrococcus, a hyperthermophilic archaeon, and additional
homologs subsequently identified in the epsilon class of the Gram-negative
bacterial phylum Proteobacteria, such as Helicobacter pylori. RESULTS: Systematic
analysis of R.PabI homologs and their neighboring genes in sequenced genomes
revealed co-occurrence of R.PabI with M.PabI homolog methyltransferase genes.
R.PabI and M.PabI homolog genes are occasionally found at corresponding
(orthologous) loci in different species, such as Helicobacter pylori,
Helicobacter acinonychis and Helicobacter cetorum, indicating long-term
maintenance of the gene pair. One R.PabI and M.PabI homolog gene pair is observed
immediately after the GMP synthase gene in both Campylobacter and Helicobacter,
representing orthologs beyond genera. The mobility of the PabI family of
restriction-modification (RM) system between genomes is evident upon comparison
of genomes of sibling strains/species. Analysis of R.PabI and M.PabI homologs in
H. pylori revealed an insertion of integrative and conjugative elements (ICE),
and replacement with a gene of unknown function that may specify a
membrane-associated toxin (hrgC). In view of the similarity of HrgC with toxins
in type I toxin-antitoxin systems, we addressed the biological significance of
this substitution. Our data indicate that replacement with hrgC occurred in the
common ancestor of hspAmerind and hspEAsia. Subsequently, H. pylori with and
without hrgC were intermixed at this locus, leading to complex distribution of
hrgC in East Asia and the Americas. In Malaysia, hrgC was horizontally
transferred from hspEAsia to hpAsia2 strains. CONCLUSIONS: The PabI family of RM
system behaves as a mobile, selfish genetic element, similar to the other
families of Type II RM systems. Our analysis additionally revealed some cases of
long-term inheritance. The distribution of the hrgC gene replacing the PabI
family in the subpopulations of H. pylori, hspAmerind, hspEAsia and hpAsia2,
corresponds to the two human migration events, one from East Asia to Americas and
the other from China to Malaysia.