BARIERY
MOLEKULARNE
W WYMIANIE GENÓW
POMIĘDZY BAKTERIAMI
Marian Sęktas
1. Wprowadzenie. 2. Przekazywanie
informacji genetycznej pomiędzy komórkami bakterii. 2.1. Pionowy
(wertykalny) transfer genów. 2.2. Horyzontalny transfer genów.
3. Metody wykrywania obszarów obcego
DNA w obrębie DNA chromosomowego. 4. Cechy obcego DNA jako
substratu procesów rekombinacji.
5. Koncepcja gatunku genomowego bakterii. 6. Podsumowanie
Molecular
barriers to interspecies gene exchange among bacteria
Abstract:
Recently,
the study of bacterial evolution has been based on the comparative
analysis of nucleotide sequences within and between species.
Analyses of microbial genomes has shown that genomes contain genes
that are closely related to phylogenetically, very distantly
related procaryotes. Usually, evolutionary biologists have thought
mutations within individual genes, followed by clonal transfer
of genetic information, is the major source of phenotypic
variation, leading to adaptation through
natural selection and generating diversity among species. Contrary
to eukaryotes, which evolve principally through the
modification of existing genetic information, bacteria obtain a
significant proportion of their genetic diversity through the
acquisition of DNA fragments from distantly related organisms.
Horizontal gene transfer is the term used to describe the
processes that permit the exchange of DNA among organisms of
different species. Such a transfer produces highly dynamic genomes
in which substantial amounts of DNA are introduced into and
deleted from the chromosome. Changes in the genome,
occurring through gene acquisition and deletion, are the major
events underlying the emergence
and evolution of a new bacterial strains including pathogens.
However, beside ecological isolation and the fitness of new
recombinants, there are several molecular barriers to
chromosomal gene transfer between bacterial species, which
correlate with genomic sequence divergence/homology. First,
specific uptake sequences (US) in DNA may be required for an
efficient transformation
process. The real barrier to successful DNA aquisition being
recipient celi are restriction-modification
systems and the lack of Chi-like sequences in the structure of
alien DNA, both of which
provide a means to species specific endo- and exonucleolytic
degradations, respectively. The strong barrier for homologous recombination is the
methyl-directed mismatch repair (MMR) system. MMR-deficient
mutators exhibit a hyperrecombinetic phenotype and act to reduce
sequence homology limitations in such processes like
transformation, transduction, and conjugation. Repeated changes in
the MMR phenotype (loss and acquisition) are involved in rapid
genetic diversity and the quick
adaptation of bacterial populations.
1. Introduction.
2. Transfer of genetic information between bacteria. 2.1. Vertical
gene transfer. 2.2. Horizontal gene transfer. 3. Methods of
detection of alien DNA regions within bacterial chromosomes. 4. Properties of DNA as a recombination
substrate. 5. Genom species concept. 6. Conclusions
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