Initiation of meiotic chromosome synapsis at centromeres in budding yeast.

Previous studies of synaptonemal complex assembly in budding yeast have suggested that chromosome synapsis initiates at the sites of crossing over. The data presented here, however, indicate that centromeric regions are preferred sites for synapsis initiation. At early times during meiosis in wild type, the Zip1 protein (a major building block of the synaptonemal complex) localizes specifically to centromeric regions. As synapsis progresses and linear stretches of Zip1 are formed, the majority of stretches are associated with a centromere, as expected if the Zip1 protein present at the centromere polymerized outward along the chromosome arm. In many cases, the centromere is present at one end of a linear stretch, suggesting that synapsis is often unidirectional. Furthermore, the Zip2 protein, a protein that promotes Zip1 polymerization, is often present at the opposite end from the centromere, implying that Zip2 and associated proteins move at the leading edge of Zip1 polymerization. Surprisingly, synapsis initiation at centromeres is independent of the Zip3 protein, which plays a major role in synapsis initiation events at noncentromeric locations. Our data provide evidence for two classes of synapsis initiation events that differ in location, timing, genetic requirements, and relationship to meiotic recombination.