Defining The Essential Function Of Lid2 Complex And Clr6 Complex In S.Pombe

Essential genes are very important for cell growth.In S.pombe,we identify that lethality of some essential genes can be suppressed by monogenic mutations which we named bypass suppressors of essential gene.Moreover,discovering and defining the role of suppressor is substantially contributed to understand function of essential gene.S.pombe jmj3 is an essential gene encoding a JmjC domain-containing protein sharing sequence similarity with histone demethylases.To explore the essential function of jmj3,we used transposon insertion mutagenesis to screen for its bypass suppressors.The suppressors included loss-of-function mutations of genes encoding Set1C/COMPASS complex subunits that are important for histone H3K4 methylation.The essentiality of Jmj3 is related to Set1C dependent H3K4me,but mutants defective in H3K4 trimethylation but not mono-and di-methylation failed to bypass jmj3.Because Jmj3 lacks conserved residues essential for demethylase activity,the main role of Jmj3 is not H3K4 demethylation,but rather counteracting the downstream toxic effect of H3K4 methylation.Through chemical mutagenesis-based screening,we found that mutations disrupting the PHD domain of Png 1 could rescue the lethality of jmj3A without affecting the level of H3K4 methylation.And Pngl can bind to H3K4me through its PHD domain and recruit NuA4 complex through its N terminal.Furthermore,we reveal that Jmj3 can block chromatin binding of Pngl and inhibit the toxicity caused by Mstl_PHD overexpression.This suggests that the essential function of Jmj3 is largely attributed to antagonize the toxic effect of NuA4C recruited by Pngl,a reader of H3K4 methylation.Except for that,Lid2,in the same complex(Lid2C)with Jmj3,has a similar essential function of Jmj3.Thus,we conclude that the essential function of Lid2C is to antagonize the toxic effect of NuA4C recruited onto chromatin by H3K4 methylation.S.pombe clr6 is also an essential gene encoding a deacetylase,required for deacetylation of multiple histone sites.However,through the bypass suppressors screens,we find that the essentiality of clr6 may not be related to the level of histone acetylation.And,there are three SIN3 homologs in Clr6 complex.Pst1 and Pst3 belong to Clr6 complex ?,Pst2 belongs to Clr6 complex ?.Moreover,genes encoding Pst1 and Pst3 are essential genes but gene encoding Pst2 is non-essential.Interestingly,we find that pst2? can rescue the lethality of pst1? or pst3?,but cannot rescue the lethality of pstl?pst3?.However,the suppressors of clr6 can rescue the lethality of pst1?pst3?pst2?.These results suggest that the main role of three SIN3 homologs may regulate the activity of Clr6 complex,but the essential function of Pst1-and Pst3-regulated Clr6 complex ? may inhibit the toxicity of Pst2-regulated Clr6 complex ?,maintaining the steady-state of Clr6 complex.In summary,through bypass-of-essential gene suppressors screens and function dissection,we gradually understand the essentiality of Lid2 complex and preliminary realize the essentiality of Clr6 and Clr6 complex.