DNA sequence of Crithidia fasciculata kinetoplast minicircles.

Kinetoplast DNA (kDNA) networks of the insect trypanosomatid Crithidia fasciculata strain CF-C1 contain a nearly homogeneous population of kDNA minicircles as judged by restriction enzyme cleavage analysis. We have determined the entire nucleotide sequence of the major class of minicircles by analyzing M13 phage clones carrying half-length segments of the kDNA minicircle molecules. The 12 nucleotide sequence d(G-G-G-G-T-T-G-G-T-G-T-A) is the longest sequence common to kDNA minicircles from several trypanosome species examined to date. Two copies of this universal minicircle sequence were identified 180 degrees apart as direct repeats within the C. fasciculata kDNA minicircles. In addition, these universal minicircle sequences are contained within direct repeats with nearly identical sequences of 173 and 177 base pairs (bp) in length. These sequences are also conserved in the same arrangement in minor sequence classes of minicircles from this strain. Site-specific discontinuities on both strands of the minicircle, identified previously in minicircle replication intermediates, were localized within the 173 and 177 bp conserved sequences. These sequences were also found to have extensive homology with similar conserved sequences in kDNA minicircles from Leishmania tarentolae. We suggest that the two conserved sequences, each containing a single copy of the universal minicircle sequence, represent replication origins in the Crithidia minicircles.