CHANGES IN PROTEIN-COMPOSITION OF MEIOTIC NODULES DURING MAMMALIAN MEIOSIS

Homologous chromosome synapsis and meiotic recombination are facilitat ed by several meiosis-specific structures: the synaptonemal complex (S C), and two types of meiotic nodules: (1) early meiotic nodules (MNs), also called zygotene nodules or early recombination nodules, and (2) late recombination nodules (RNs), The former are thought to be nucleop rotein complexes involved in the check for homology preceding, or acco mpanying synapsis, while the latter have been shown to be involved in reciprocal recombination, We have examined by immunocytochemistry the meiotic localization of a series of proteins at sites along the asynap sed axial elements prior to homologous synapsis and at sites dong the SCs following synapsis, Several of the proteins examined have been imp licated in repair/recombination and include RAD51, a mammalian homolog of the Escherichia coli RecA protein; Replication Protein-A (RPA), a single-strand DNA binding protein; and MLH1, a mismatch repair protein which is a homolog of the E. coli MutL protein, In addition two prote ins were examined that have been implicated in meiotic checkpoints: AT M, the protein mutated in the human disease Ataxia Telangiectasia, and ATR, another member of the same family of PIK kinases, We present evi dence that these proteins are all components of meiotic nodules and do cument changes in protein composition of these structures during zygon ema and pachynema of meiotic prophase in mouse spermatocytes. These st udies support the supposition that a subset of MNs are converted into RNs, However, our data also demonstrate changes in protein composition within the context of early MNs, suggesting a differentiation of thes e nodules during the process of synapsis, The same changes in protein composition occurred on both the normal X axis, which has no homologou s pairing partner in spermatocytes, and on the axes of aberrant chromo somes that nonhomologously synapse during synaptic adjustment, These f indings suggest that DNA sequences associated with MNs still must unde rgo an obligatory processing, even in the absence of interactions betw een homologous chromosomes.