Structure and mitotic stability of minichromosomes originating in yeast cells transformed with tandem dimers of CEN11 plasmids.

Large (10.5-13.5 kbp) circular minichromosomes containing the centromere of chromosome 11 (CEN11) and the MET14 gene of Saccharomyces cerevisiae in the YRp7 vector are considerably more stable during mitosis than smaller ones containing only the 1.6 kbp CEN11 SalI-fragment. Yeast transformants obtained with a tandem dimeric and thus dicentric form derived from this DNA varied in the mitotic stability of the TRP1 marker of the vector. The largest group of transformants contained minichromosomes which carried deletions located quite specifically at one of the two centromeres in the dimer, eliminating its function in mitosis. This group included also some minichromosomes which had been modified by intramolecular tandem amplification of the subunit carrying the deletion without losing the centromere within the unmodified subunit. The second major group carried minichromosomes which had been monomerized. Monomerized minichromosomes showed the relative low degree of mitotic stability typical for the original minichromosomes containing the 1.6 kbp CEN11 SalI-fragment. Increasing numbers of additional subunits carrying the TRP1-ARS1 sequences but lacking additional centromeres improved the mitotic stability considerably.