Regulation of Sum1 stability in response to pachytene checkpoint

In budding yeast, cells defective in meiotic recombination undergo checkpoint-mediated arrest at the pachytene stage. The molecular mechanism of the pachytene checkpoint machinery in controlling meiotic cell cycle is not clear. It is known that Ndt80 and Sum1 are involved in the arrest. Ndt80 is a meiosis-specific transcription activator of middle sporulation genes, and it plays a critical role in regulating the progression during meiosis. Triggering of pachytene checkpoint prevents the accumulation of active Ndt80. Sum1 is a transcription repressor which inhibits the expression of middle sporulation genes, including the NDT80, at the early stage of meiosis. The level of Sum1 protein decreases transiently in wild-type cells during meiosis. In the pachytene-arrested cells, the level of Sum1 protein is stabilized, indicating that the stability of Sum1 protein is regulated by the pachytene checkpoint. Two different models have been proposed for the regulation of Sum1 stability by the pachytene checkpoint. The pachytene checkpoint may directly control the stability of Sum1. Alternatively, the stability of Sum1 is indirectly regulated through the activity of Ndt80 protein. To distinguish between these two models, the stability of Sum1 protein was analyzed in wild-type, dmc1, and dmc1 NDT80-bc cells. NDT80-bc is a dominant allele, which can completely bypass the pachytene arrest. Consistent with the previous study, the level of Sum1 decreased as the progress of meiosis in wild-type cells. In the dmc1 mutant, the pattern of Sum1 protein level was relatively constant during meiosis. In the dmc1 NDT80-bc strain, although the pachytene checkpoint was activated, the level of Sum1 protein was still declined in the presence of active Ndt80-bc protein. These data indicated that the Sum1 protein is probably not regulated directly by the pachytene checkpoint, and its stability might be controlled by the Ndt80 activity.