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 an

other 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 establish

ment 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 inactive1. 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 activity2. a conserved acetyltransferase( 乙酰基转移酶 ) activity, which triggers the gener

ation 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