Fig. 2.

Chromosome structure surrounding centromere 3 (cen3) of S. pombe. The 4.9-kb central element (cnt) is flanked by 5.4-kb inverted innermost repeats (imr), which in turn are flanked by multiple 6.7-kb outermost repeats (otr) (35). The number of outermost repeats to the left of the central element is uncertain, but we estimate there are 6 ± 1. For genetic analysis the ura4⁺ gene was substituted for most of chk1, and his3⁺ was inserted between mid1 and the next centromere-distal gene cwf20; chk1 and mid1 are protein-coding genes separated from centromere 3 by, respectively, one and three protein-coding genes (35). Recombination between chk1::ura4⁺ and mid1-322::his3⁺ is used as a measure of centromere 3 recombination (Table 1). The ade6 gene is 166 kb from the mid1-322::his3⁺ insertion; recombination between these markers is used as a measure of cen3–ade6 recombination. Two ade6 markers, ade6-M26 and ade6-52, separated by 0.66 kb, are used to measure ade6 intragenic recombination. The small rectangle (bottom left) indicates the position of the radioactive probe for the Southern blot hybridizations of the indicated BglI DNA fragment (Fig. 3 and Table 1). The figure is drawn approximately to scale, except for the position ade6.