Horizontal Gene Transfer


What is Horizontal Gene Transfer?

Natural genetic transformation is believed to be the essential mechanism for the attainment of genetic plasticity in many species of bacteria. During bacterial evolution, the ability of Bacteria and Archaea to adapt to new environments most often results from the acquistion of new genes through horizontal transfer rather than by the alteration of gene functions through numerous point mutations. Horizontal gene transfer is defined to be the movement of genetic material between bacteria other than by descent in which information travels through the generations as the cell divides. It is most often thought of as a sexual process that requires a mechanism for the mobilization of chromosomal DNA among bacterial cells. However, because they are unable to reproduce sexually, bacterial species have acquired several mechanisms by which to exchange genetic materials.

  1. Transformation - the uptake of naked DNA is a common mode of horizontal gene transfer that can mediate the exchange of any part of a chromosome; this process is most common in bacteria that are naturally transformable; typically only short DNA fragments are exchanged.
  2. Conjugation - the transfer of DNA mediated by conjugal plasmids or conjugal transposons; requires cell to cell contact but can occur between distantly related bacteria or even bacteria and eukaryotic cells; can transfer long fragments of DNA.
  3. Transduction - the transfer of DNA by phage requires that the donor and recipient share cell surface receptors for phage binding and thus is usually limited to closely related bacteria; the length of DNA transferred is limited by the size of the phage head.
Each of these methods of genetic exchange can introduce sequences of DNA that share little homology with the remaining DNA of the recipient cell. If there are homologous sequences shared between the donor DNA and the recipient chromosome, the donor sequences can be stably incorporated into the recipient chromosome by genetic recombination. If the homologous sequences flank sequences that are absent in the recipient, the recipient may acquire an insertion from another strain of unrelated bacteria. Such insertions can be small or quite large. Large insertions that have been acquired from another bacterium (often inferred from differences in GC content or codon usage) and are absent from related strains of bacteria are called "islands".

Mosaic Genes

A common way that Bacteria and Archae seem to different environmental conditions is via the acquisition of mosaic genes. A mosaic gene is:

  1. an allele that has acquired, through tranformation and subsequent integration into the original allele, a DNA sequence from a different species of bacteria;
  2. composed of sequence polymorphisms identical to the original allele in some parts of the gene but polymorphisms derived from the integrated DNA in other parts.
Often, the integrated sequence contains a selectable genetic marker, such as antibiotic resistance. Therefore, horizontal exchange through transformation permits movement of alleles in bacterial lineages, resulting in mosaics that increase the opportunities for the spread of such genes as those that encode for antibiotic resistance. There are many examples of horizontal genetic exchange through both transformation and conjugation in bacteria species that have enabled them to better adapt to their environment.

One of the most well-characterized examples of mosaic genes are those that encode the penicillin-resistant binding proteins found in Streptococcus pneumoniae. These high molecular weight proteins are the lethal targets of the B-Lactams of penicillin. Therefore, some species have acquired modified proteins with low affinity for the B-lactams, resulting in a form of target-mediated anitbiotic resistance. This type of resistance occurs when the species acquires a variant of the particular target of the antibiotic that has a normal or near-normal metabolic function but a low binding-affinity for the antibiotic.

These modified proteins are most likely obtained through intragenic recombination as a result of transformation events. This results in novel alleles that are mosaics containing genes from other streptococcal species that encode resistant proteins.

Therefore, these resistant strains that carry the mosaic allele express a phenotype that is favored by selection and the frequency of such genes that encode for antibiotic resistance tends to rise in a given population.

Genetic Islands

Another way that Bacteria and Archae can adapt to new environmental conditions is via the acquisition of large contiguous fragments of DNA. Such large DNA fragments may be acquired in several ways:

Regions of the chromosome that seem to be acquired by the inheritance of long DNA sequences can be identified by comparison of genomes from closely related bacteria -- a region of DNA that is present in one strain of bacteria but lacking in other closely related strains indicates that either an insertion occured in the strain with the sequence or a deletion of that sequence occured in the other strain. These regions are called "indels" or "islands" (sometimes shorter regions with these properties are called "islets").

Inheritance of many genes simultaneously allows the inheritance of complex characteristics in a single step. Thus, phenotypes that require multiple steps often to be acquired in this way. Examples include "pathogenesis islands" (PAI) and islands that enable complex metabolic pathways (such as the genes required for vitamin B12 biosynthesis in Salmonella).

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Last modified July 15, 2002