DNA Double-Strand Breaks Trigger Genome-Wide Sister-Chromatid
Cohesion Through Eco1(Ctf7)
金漫 王晓艳 沙龙泽
2007-11 -20
------ 《 Science 》 13 July 2007
what factor acts here to bind sister-chromatid together?
Cohesin
It is a model of cohesin
The difference between two key words in this paper:
Cohesin
Cohesion( 粘连过程 ):
is a process( 过程 )
Today we know
The cohesin have two functions:
① Faithful chromosome segregation ( 染色体间的分离 )
② repair of DNA double-strand breaks(DSBs)
① is induced by DNA replication
② is induced by DSB
Function 1: Faithful chromosome segregation
Arm cohesin
Centromere cohesin
S-Cohesin The word “S” means S-phase of the cell cycle
Function 2: repair of DNA double-strand breaks(DSBs)
This kind of cohesin is induced by DSBs
The DSB-cohesin is used here to repair the DNA damage
DSB-cohesin
Remember :
Both S-cohesin and DSB-cohesin
are the same kind
Induction pathway is different
Questions
• 1 Whether DNA replication is required for DSB-induced cohesion? (Relationship)
• 2 By the induce of DSBs. cohesion occur only around broken chromosomes and not on unbroken chromosome?
• 3 Eco1(cohesion establishment factor) ,How to mediate the process of cohesion?
Experiment 1:The cohesin which is induced by DNA replication has a thermo-sensitive( 温度敏感型突变 ) subunit mcd1-1
Wild-type
• Actually the result is there is only one dot
• So it can be identified that the DSBs is a another pathway can induce cohesion
Why not 100% cohesion?
LossGo back to 20%
Up to 60%
Conclusion of Experiment 1
• 1 Cohesion can be generate outside of S-phase
• 2 Both DSBs and wild-type cohesion(S-cohesin) are necessary, but neither is sufficient
• 3 As few as two DSBs are sufficient to induce cohesin-dependent cohesion in G2/M
Questions
• 1 Whether DNA replication is required for DSB-induced cohesion? (Relationship)
• 2 By the inducement of DSBs,whether cohesion can occur only around broken chromosomes and not on unbroken chromosomes?
• 3 What is the key factor for the cohesion establishment?
Experiment 2• We move the cohesion reporter (LacI-GFP) to chr.Ⅰ
while keeping the HO-cs on chr . We found that thouⅢgh there is no DSB on chr. , cohesion is still establisheⅠd by the induction of DSBs on chr. Ⅲ
Conclusion of experiment 2
• Cohesion can occur both around broken chromosomes and on unbroken chromosomes genome-wide cohesion
• This generation of genome-wide cohesion cannot occur by a replication-dependent mechanism
• DSBs can trigger genome-wide sister-chromatid cohesion
Background of Experiment 3
• DNA damage response pathway
• Key protein: MRE11 MEC1 H2A TEL1
Nucleosome(DNA+histon
e)
Experiment 3
• We examined damage-induced cohesion in cells mutated for MRE11, MEC1 , H2AX and TEL1.
Results of experiment 3• 1.In mer11 or △
mec1 cells, coh△esion fails to form on chr. in resⅩⅥponse to DSBs on chr.Ⅲ.( means △absent)
2.In Tell and H2A mutated cells, the loss rate remains the s△ame as thee wild-type cell.
Conclusion of experiment 3
• The chromatin-bound cohesion complex is converted to a cohesive state by the DNA damage response pathway(mer11and mec1 are necessary), presumably through a trans-acting factor.
Questions
• 1 Whether DNA replication is required for DSB-induced cohesion? (Relationship)
• 2 By the inducement of DSBs,whether cohesion can occur only around broken chromosomes and not on unbroken chromosomes?
• 3 What is key factor for the cohesion establishment? (the trans-acting factor)
One candidate
• Eco1 An essential factor for cohesion establis
hment in S phase.
• Now we will do another experiment to identify whether Eco1 is also the key factor for DSB-induced cohesion establishment in G2/M.
Experiment 4
• S-cohesion inactive
1. without either DSB or Eco1,loss rate is high
2. with both DSB and Eco1,loss rate is lower
Experiment 4
• S-cohesion inactive Eco1 ack- acetyltransferase ( 乙酰基转移酶 )
• - or + DSB, loss rates are both high,and almost the same
Conclusion of experiment 4
So Eco1 is the key factor for DSB-induced cohesion and the acetyltransferase ( 乙酰基转移酶 ) is the key domain.
Ecol has at least two distinct biological functions:
• Converts the chromatin-bound cohesion complex to a cohesive state
• The acetyltransferase function activates directly or indirectly its cohesive function during G2/M.
Summary 1
• Cohesion is generated by an Eco1-dependent but replication-independent process in response to DSBs in G2/M
Summary 2
• Eco1 has two functions:
1. a cohesive activity
2. a conserved acetyltransferase( 乙酰基转移酶 ) activity, which triggers the generation of cohesion in response to the DSB and the DNA damage checkpoint.
Summary 3
• The DSB-induced cohesion is not limited to broken chromosomes but occurs also on unbroken chromosomes, suggesting that the DNA damage checkpoint through Eco1 provides genome-wide protection of chromosome integrity.
Conclusion
Replication fork
Cohesion is generated by the replication-driven process and Eco1 ;Eco1 can directly converts the cohesin to its cohesive state,making the sister-chromatides get together
With DSB,the DNA damage checkpoint initiates a signal that induces wild-type cohesin loading around the DSB and the Eco1 is reactive.In response to DSBs,the Eco1 acetyltransferase activity is increased through the DNA damage checkpoint.
Furthermore• Pay more attention to:1. Where is the acetyltransferase domain? 2. What are the inhibitors that limit the activity
of Eco1 in G2/M on unbroken chromosomes?3. How does Eco1 work in S-phase?
• Further studies of genome-wide cohesion may reveal additional functions such as the preventions of rearrangements through ectopic recombination
Remainunclear now