THE RAD9 GENE OF COPRINUS-CINEREUS ENCODES A PROLINE-RICH PROTEIN REQUIRED FOR MEIOTIC CHROMOSOME CONDENSATION AND SYNAPSIS

The rad9 gene of Coprinus cinereus is essential for the normal complet ion of meiosis. We examined surface-spread preparations of wild-type a nd rad9-1 nuclei from the meiotic stages of karyogamy through metaphas e I, and we determined the primary sequence, structure, and meiotic ex pression of the rad9 gene. In wild-type C. cinereus, karyogamy is foll owed by condensation and alignment of homologous chromosomes. Condensa tion and axial core development largely precede synapsis, which often initiates at telomeres. A diffuse diplotene phase coincides with disso lution of the synaptonemal complex, and subsequently chromosomes furth er condense as the cells progress into metaphase I. In contrast, altho ugh karyogamy and nucleolar fusion are apparently normal in rad9-1 bas idia, only short stretches of synaptonemal complex form. These correla te with stretches of condensed chromatin, mostly at apparent chromosom e ends, and regions of presumptive triple synapsis are numerous. rad9- 1 basidia enter the diffuse stage of early diplotene, and then 50% of these cells enter metaphase I by the criteria of nucleolar elimination and at least some chromatin condensation. rad9 gene expression is ind uced after gamma irradiation and during meiosis. The gene has 27 exons and encodes a predicted protein of 2157 amino acids, with a proline-r ich amino terminus.