A role for the Saccharomyces cerevisiae Rtt109 histone acetyltransferase in R-loop homeostasis and associated genome instability

The stability of the genome is occasionally challenged by the formation of DNA-RNA hybrids and R-loops, which can be influenced by the chromatin context. This is mainly due to the fact that DNA-RNA hybrids hamper the progression of replication forks, leading to fork stalling and, ultimately, DNA breaks. Through a specific screening of chromatin modifiers performed in the yeast Saccharomyces cerevisiae, we have found that the Rtt109 histone acetyltransferase is involved in several steps of R-loop-metabolism and their associated genetic instability. On one hand, Rtt109 prevents DNA-RNA hybridization by the acetylation of histone H3 lysines 14 and 23, and on the other hand, it is involved in the repair of replication-born DNA breaks, such as those that can be caused by R-loops, by acetylating lysines 14 and 56. In addition, Rtt109 loss renders cells highly sensitive to replication stress in combination with R-loop-accumulating THO-complex mutants. Our data evidence that the chromatin context simultaneously influences the occurrence of DNA-RNA hybrid-associated DNA damage and its repair, adding complexity to the source of R-loop-associated genetic instability.基因组稳定性偶尔会受到DNA-RNA杂交体及R环（R-loops）形成的挑战，而这一过程可受染色质环境的影响。究其根本原因，DNA-RNA杂交体会阻碍复制叉的行进进程，引发复制叉停滞，最终导致DNA断裂。我们通过在酿酒酵母（Saccharomyces cerevisiae）中开展的染色质修饰因子特异性筛选实验，发现Rtt109组蛋白乙酰转移酶（histone acetyltransferase）参与了R环代谢的多个步骤及其相关的遗传不稳定性调控过程。一方面，Rtt109可通过乙酰化组蛋白H3的赖氨酸14与23位点，阻止DNA-RNA杂交体的形成；另一方面，它可通过乙酰化赖氨酸14与56位点，参与修复由R环诱发的复制源性DNA断裂。此外，在R环积累的THO复合物（THO-complex）突变体背景下，Rtt109的缺失会使细胞对复制应激表现出高度敏感性。本研究数据证实，染色质环境可同时影响DNA-RNA杂交体相关DNA损伤的发生及其修复过程，进一步增加了R环相关遗传不稳定性来源的复杂性。