One strand of ars189 from the maxicircle of Crithidia fasciculata, transforms Saccharomyces cerevisiae more efficiently than its complementary strand as a single stranded DNA

An autonomously replicating element (ars 189) has been isolated from the maxicircle DNA of an insect trypanosomatid Crithidia fasciculata. This 189-bp fragment contains two copies of the yeast consensus ARS sequence of (A/T)TTTATPuTTT(T/A), has an A + T composition of 79.4%, and shows a large asymmetry in the distribution of adenine and thymine residues between the two strands. The complementary strands of ars 189 have been cloned into an M 13 vector containing the URA3 gene of Saccharomyces cerevisiae. When these circular single-stranded (ss) DNAs were used to transform yeast spheroplasts, the M 13 chimeric DNA carrying the strand of ars189 rich in adenine generated approximately four times more yeast Ura⁺ transformants than the construct containing the thymine-rich strand. In contrast, both strands of yeast ARS1 cloned into an M 13 vector transformed yeast at an equivalent level. The conversion of ARS -containing ss DNAs to duplex forms in vivo and their subsequent autonomous replication have been verified by Southern hybridization analysis of extracts from yeast transformants.