Characterization of Schizosaccharomyces pombe minichromosome deletion derivatives and a functional allocation of their centromere.

A 530 kb long Schizosaccharomyces pombe linear minichromosome, Ch16, containing a centric region of chromosome III, has previously been made. In the present study, we constructed a number of deletions in the right and/or left arms of Ch16, and compared their structure and behaviour with Ch16. The functional centromere, cen3, is allocated within a 120 kb long region which is covered by the shortest derivative, Ch10, and is comprised mostly of centromeric repeating sequences. The shortest minichromosome is stable in mitosis and the copy number control is apparently precise. In monosomic meiosis it segregates normally. In disomic meioses, however, the frequency of non-disjunction is very high, suggesting that it may not form a pair. The mitotic loss rate of one of the left-arm deletions, ChR32, which lacks a part of the centromeric repeating sequence, is the highest of all the deletions. This deletion also exhibits the highest precocious sister chromatid separation in meiosis I, suggesting that sister chromatid association might become weakened in ChR32. Our results indicate that the proper meiotic segregation of S.pombe minichromosomes is dependent upon the formation of a bivalent. S.pombe may not have the 'distributive segregation' found with Saccharomyces cerevisiae minichromosomes.