Homogeneous and heterogeneous mini-circle subpopulations in Trypanosoma cruzi kinetoplast DNA.

The small circular components (mini-circles) from Trypanosoma cruzi kinetoplast DNA (kDNA) were cloned in the plasmid vector pBR325. These clones have been used before to demonstrate the rapid evolution of mini-circle subpopulations (S anchez , D.O., Frasch , A.C.C., Carrasco , A.E., Gonzalez Cappa , S.M., Isola , E. and Stoppani , A.O.M. (1984) Mol. Biochem. Parasitol ., in the press). We have now analyzed the cloned molecules and used them to study some structural characteristics of T. cruzi mini-circles and their distribution in total kDNA restriction endonuclease digests. Most molecules partially conserved TaqI, HaeIII and HapII site clusters (constant regions) separated by one-quarter of the total mini-circle length, also detected in total kDNA digests. In addition, in one of the cloned mini-circles, the constant region was present only once, instead of four times as expected. Outside the conserved regions, the mini-circles diverged enough so that no cross-hybridization took place even under relaxed conditions. The recombinant molecules were used to probe total kDNA digests from T. cruzi. Some of them hybridized with most restriction endonuclease kDNA fragments, while one cloned mini-circle ( pTck -14) detected only its homologous subpopulation. The mini-circles detected with the latter probe proved to be nearly homogeneous, and were present in the proportion of 1/20 molecules. These results suggest that some of the generated molecules might have acquired a higher replication rate, giving rise to the homogeneous subpopulation detected. Further mutations, insertions and/or deletions, together with recombination between molecules, would bring this process to an end.