During the meiotic cell cycle, a checkpoint prevents progression
into meiosis I until recombination is completed, thus ensuring the
proper segregation of chromosomes. The ZIP1 gene of S.
cerevisiae encodes a component of the central region of the
synaptonemal complex. In the zip1 mutant, chromosomes fail to
synapse and undergo a reduced level of crossing over, resulting in cell
cycle arrest at the pachytene stage of prophase I. PCH2 is a
meiosis-specific gene identified in a genetic screen for checkpoint
mutants that relieve the meiotic arrest of zip1 . Sporulation
frequency, crossing over and spore viability are not significantly
affected in the pch2 mutant. The zip1 pch2 double mutant
displays the wild-type level of sporulation, but spore viability is
decreased due to the absence of Zip1. Most of the Pch2 protein localizes
to the unsynapsed nucleolar region (rDNA), and a small fraction is found
in a punctate pattern along synapsed chromosomes overlapping with Zip1.
Pch2 is required for the repression of meiotic recombination in the
rDNA. The silencing factor Sir2 is also required for this repression.
Localization of Pch2 to the nucleolus depends on Sir2, and sir2
also bypasses the meiotic arrest of zip1 . The recombination
protein Hop1, which is excluded from the rDNA in wild type, is found in
the nucleolus in the pch2 and sir2 mutants. Possible
relationships between the nucleolus and meiotic checkpoint control will