Paused or stalled replication forks are major threats to genome integrity; unraveling the complex pathways that contribute to fork stability and restart is crucial. Experimentally, fork stalling is induced by growing the cells in presence of hydroxyurea (HU), which depletes the pool of deoxynucleotide triphosphates (dNTPs) and slows down replication ...

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progression in yeast. Here, I report an epistasis analysis, based on sensitivity to HU, betweenCLB2, the principal mitotic cyclin gene inSaccharomyces cerevisiae, and genes involved in fork stability and recombination.clb2Δ cells are not sensitive to HU, but the strong synergistic effect ofclb2Δ with most genes tested indicates, unexpectedly, thatCLB2has an important role in DNA replication, in the stability and restart of stalled forks, and in pathways dependent on and independent of homologous recombination. Results indicate thatCLB2functions in parallel with theSGS1helicase andEXO1exonuclease to allow proper Rad51 recombination, but also regulates a combined Sgs1-Exo1 activity in a pathway dependent on Mec1 and Rad53 checkpoint protein kinases. The data argue that Mec1 regulates Clb2 to prevent a deleterious Sgs1-Exo1 activity at paused or stalled forks, whereas Rad53 checkpoint activation regulates Clb2 to allow a necessary Sgs1-Exo1 activity at stalled or collapsed forks. Altogether, this study indicates that Clb2 regulates the activity of numerous nucleases at single-stranded gaps created by DNA replication. A model is proposed for the function and regulation of Clb2 at stalled forks. These data provide new perspectives on the role of mitotic cyclins at the end of S phase.

Date: Feb. 02, 2018

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