A mutant plasmid with increased stability of holding and polymerization in Saccharomyces cerevisiae

A yeast mutant plasmid, pX, showing increased stability of holding in mitotic and meiotic cell division, was isolated from an unstable plasmid, YRp7, which consists of pBR322 and a 1.4 kilobase (kb) fragment of Saccharomyces cerevisiae carrying the TRP1 gene and the autonomously replicating sequence, ARS1. The pX plasmid exists as circular molecules in a series of polymeric forms from the monomer to the 20-mer or more, consisting, except for the monomer, of even numbers of unit molecules in tandem arrangement. The pX monomer consists solely of a yeast DNA fragment of 849 base pairs (bp) containing the TRP1 and ARS1 sequences derived from the 1.4 kb yeast fragment of YRp7. The copy number of pX was calculated to be 20 as monomer units per genome. The ARS1 region was delimited to 80 bp by this mutation and the region essential for the ARS function was discussed.