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 (ars189) 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 ars189 have been cloned into an M13 vector containing the URA3 gene of Saccharomyces cerevisiae. When these circular single-stranded (ss) DNAs were used to transform yeast spheroplasts, the M13 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 M13 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.