ADVERTISEMENTS:
In this article we will discuss about the meaning of site-specific recombinases of bacteria.
During evolution of multiple antibiotic resistance, bacteria seem to have made use of enzymes known as site-specific recombinases. Each type of site-specific recombinase binds with a specific nucleotide sequence in double stranded DNA. When the site is present in each of the two duplex molecules, the recombinase brings the sites together and catalyses a reciprocal exchange between the two duplex molecules.
Site-specific recombinases are involved in the assembly of multiple antibiotic resistance units called integrons. An integron is a DNA element that encodes a site-specific recombinase and also has a recognition region that allows other sequences with similar recognition regions to be incorporated into the integron by recombination.
ADVERTISEMENTS:
The elements that integrons acquire are known as cassettes. With respect to integrons, a cassette is a circular antibiotic-resistance-coding region flanked by a recognition region for an integron. Because the site-specific recombinase integrates cassettes, the integron recombinase is usually called an integrase.
In general, antibiotic-resistance cassettes contain protein-coding regions but do not have the promoter sequences that are required for initiation of transcription. They can be transcribed only by read-through transcription from an adjacent promoter. The integron provides the required promoter called Pant, so that when a cassette is captured, the coding sequence can be expressed.
More than 40 different promoter-less cassettes have been described that encode proteins for resistance to antibiotics. Integrons by themselves are not capable of mobility. They are present in transposons, conjugative plasmids, non-conjugative plasmids as well as bacterial chromosomes.
Integrons that are present in mobile DNA elements are particularly important in the evolution of antibiotic resistance because they can capture antibiotic-resistance cassettes, and thereby make possible transcription of the antibiotic resistance coding sequences as well as their mobilisation.
ADVERTISEMENTS:
Bacterial Resistance to Antibiotics and Public Health:
Over a period of time, a conjugative plasmid can accumulate different transposons containing multiple independent antibiotic resistance genes. The plasmid could also contain transposons having integrons that have acquired multiple antibiotic resistance cassettes. Such a plasmid is capable of conferring resistance to a large number of unrelated antibiotics.
Such multiple resistance plasmids are called R plasmids. The development and evolution of R plasmids is associated with the use of antibiotics. Evolution selects for resistant cells because in the presence of antibiotics, growth of resistant cells has an advantage over that of cells that are sensitive.
In the presence of multiple antibiotics, there is selection in favour of multiple drug resistance. When plasmids resistant to multiple drugs are transferred to pathogenic bacteria that cause human diseases, it results in serious clinical complications. Infections caused by pathogens that contain R factors are difficult to treat owing to pathogen’s resistance to available antibiotics.
