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However, this structure is relatively rare, and most class 1 integrons are associated with an incomplete transposition ( (1, 25, 42, 49).
Class 1 integrons, therefore, are found in association with considerable structural diversity, especially in relation to the makeup of the 3′-CS (4, 14, 34, 35).
Here, we have isolated bacteria from soil and sediment in the absence of antibiotic selection.
Class 1 integrons were recovered from four different bacterial species not known to be human pathogens or commensals.
These inverted repeats therefore facilitate the movement of class 1 integrons by transposition and define the limits of the mobile unit that includes the class 1 integron in a structural sense.
Transposition of a class 1 integron requires four transposition genes of the type found in Tn-like transposition system.
After that time, resistance genes were commonly observed to be present on mobile elements, such as plasmids and transposons; before that time, they appear to have been relatively rare on these elements (18).
The antibiotic era also coincided with a time of rapid appearance of class 1 integrons carrying antibiotic resistance genes, with these integrons appearing in a number of independent locations in different transposons and plasmids (28, 48).
If class 1 integrons were originally derived from a chromosomal integron, then association with transposition genes that facilitate their mobility presumably occurred in the preantibiotic era or very early after the onset of the antibiotic era.Despite these multiple rearrangements, all retain evidence of at least some Tn transposition functions was an ancient event or one that occurred in historical times, it is nonetheless the case that it has become a powerful vehicle for the spread of resistance genes.The site-specific recombination system has allowed individual class 1 integrons to capture a diverse array of resistance determinants.In contrast, “chromosomal” integrons are capable of recruiting a diverse assortment of novel mobile genes but are, nonetheless, confined to defined phylogenetic lineages (8, 17, 31, 46).Antibiotic resistance as a problem arose soon after the onset of the clinical use of antibiotics.
Class 1 integrons are generally embedded in mobile elements, including numerous plasmids and transposons.