| Abstract
| - The protozoan parasite Leishmania often responds to drug pressure by amplifying part of its genome. At least two loci derived from the same 800 kb chromosome were amplified either as extrachromosomal circles or linear fragments after sodium arsenlte selection. A 50 kb linear amplicon was detected in six Independent arsenlte mutants and revertants grown in absence of arsenlte rapidly lost the amplicon and part of their resistance. The circular extrachromosomal ampllcons, all derived from the H locus of Leishmania, were characterized more extensively. In all cases, direct repeated sequences appeared to be Involved in the formation of circular ampllcons. Most amplicons were generated after homologous recombination between two linked P-glycoproteln genes. This recombination event was, in two cases, associated with the loss of one allele of the chromosomal copy. A novel rearrangement point was found in a mutant where the amplicon was created by recombination between two 541 bp direct repeats surrounding the P-glycoproteln gene present at the H locus. It Is also at one of these repeats that an H circle with large inverted duplications was formed. We propose that the presence of repeated sequences in the H locus facilitates the amplification of the drug resistance genes concentrated In this locus.
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