Defining response pathways of budding yeast checkpoint genes

Abstract

Cell cycle events are ordered correctly; mitosis follows DNA replication. To ensure correct order, cells employ checkpoints that delay the cycle when DNA replication, repair, or spindle assembly have not been completed. In this dissertation, I have focused on the DNA damage checkpoint, which arrests the cell in G₂ in response to DNA damage (the G₂/M checkpoint). I have studied the roles of several checkpoint genes in the budding yeast Saccharomyces cerevisiae involved in the response to DNA damage, focusing on a key gene called MEC1. I have tested genetically where in several pathways checkpoint genes act: G₂/M checkpoint pathway. I found that after DNA damage, MEC1 signals G₂/M cell cycle arrest using two pathways, one involving RAD53 and DUN1, and the other involving PDS1. Both pathways must be functional for full checkpoint arrest; either pathway acting alone produces only a partial arrest. I speculate why there are two pathways for arrest. TEL1. I also tested the roles of TEL1, a putative MEC1 homolog. I showed that TEL1 has no normal checkpoint function. However, when overexpressed, TEL1 produces a constitutive G₂ delay, independent of DNA damage, a delay that requires the PDS1 pathway. This constitutive delay is responsible for the suppression by TEL1 of the UV sensitivity of mec1 mutants. When overexpressed, TEL1 also restores damage-inducible transcription to mec1 cells. I discuss TEL1's possible roles in checkpoint mediated responses. Essential function pathway. Previous results showed that MEC1 and RAD53 are also required for the transcriptional induction of repair genes and for an essential function. The nature of their essential function(s) remains unknown. My results, from a complex series of genetic tests, suggest that MEC1 and RAD53 share the same essential function, and that this function may in fact be related to the transcriptional function. I speculate on the nature of the essential function. I also present evidence that MEC1 and RAD53 may have a role in DNA replication. My results have led to refined models of pathways leading to checkpoint arrest, damage-inducible transcription, and an essential function(s).