Bubble-shaped adaptor element and method of constructing sequencing library with bubble-shaped adaptor element

1. An adaptor element for constructing a sequencing library, being a hybrid formed with a long-chain nucleic acid A and a short-chain nucleic acid B, wherein

the long-chain nucleic acid A consists of a fragment A1 at the 3′-terminal, a fragment A2 in the middle and a fragment A3 at the 5′-terminal, and

the short-chain nucleic acid B consists of a fragment B1 at the 5′-terminal, a fragment B2 in the middle and a fragment B3 at the 3′-terminal, wherein

part of the fragment A1 at the 3′-terminal and the fragment B1 at the 5′-terminal forms a first paired region, the fragment A3 at the 5′-terminal and the fragment B3 at the 3′-terminal forms a second paired region, and the fragment A2 in the middle and the fragment B2 in the middle forms a non-paired region, the fragment A3 of the long-chain nucleic acid A has a phosphorylated base at the 5′-terminal and the fragment B3 of the short-chain nucleic acid B has one overhang at the 3′-terminal, wherein the overhang is a single Thymine (T) base,

the second paired region contains a recognition site of class III restriction endonuclease, and the recognition site of class III restriction endonuclease is 0-2 bp away from where the adaptor element is ligated to a target DNA fragment,

the fragment B1 contains a site of Uracil (U) or deoxyuridine (dU), recognizable and digestible by a uracil-specific excision reagent, and

the long-chain nucleic acid A has a length longer than that of the short-chain nucleic acid B.

2. The adaptor element according to claim 1 , wherein the hybrid comprises three regions:

a first region, comprising the first paired region and a non-paired single strand at the 3′-terminal of the long-chain nucleic acid A,

a second region, being the non-paired region which is in a bubble shape consisting of two non-paired single strands, and

a third region, comprising

the second paired region and the overhang at the 3′-terminal of the short-chain nucleic acid B.

3. The adaptor element according to claim 1 , wherein the long-chain nucleic acid A comprises the sequence of 5′-/Phos/CTGCTGACGTACTGTGTCATAAATAGCACGAGACGTTCTCGACT/ddC/-3′ (SEQ ID NO:1), where /Phos/ represents a phosphorylation modification, /ddC/ represents dideoxycytidine, and

the short-chain nucleic acid B comprises the sequence of 5′-GAGAACGUCTCGTGCUACGTTCTCGACTCAGCAGT-3′ (SEQ ID NO:2).

4. The adaptor element according to claim 1 , wherein the long-chain nucleic acid A comprises the sequence of 5′-/Phos/AGTCGGAGGCCAAGCGTGCTTAGGACAT-3′ (SEQ ID NO:3), where /Phos/ represents a phosphorylation modification, and

the short-chain nucleic acid B comprises the sequence of 5′-GTCCTAAGCACUGTAGTGTACGATCCGACTT-3′ (SEQ ID NO:4).

5. A method of ligating an adaptor element, comprising:

subjecting a DNA fragment of interest to end-repairing, phosphorylation at the 5′-terminal and adenine (A) addition to the 3′-terminal prior to ligating the adaptor element as defined in claim 1 ,

and

ligating the adaptor element to the resulting DNA fragment respectively at two terminals.

6. A kit for constructing a sequencing library, comprising the adaptor element as defined in claim 1 .

7. The adaptor element according to claim 1 , wherein the length of the fragment A1 is 10 nt to 20 nt and the length of the fragment B1 is 8 nt to 16 nt,

the length of the fragment A2 is 11 to 36 nt and the length of the fragment B2 is 11 nt to 21 nt, and

the length of the fragment A3 is 6 nt to 8 nt and the length of the fragment B3 is 7 nt to 9 nt.

8. The adaptor element according to claim 1 , wherein the fragment A2 comprises a barcode sequence.

9. The adaptor element according to claim 1 , wherein the class III restriction endonuclease is Acu I, Bpm I, BceA I, Bbv I, BciV I, BpuE I, BseM II, BseR I, Bsg I, BsmF I, BtgZ I, Eci I, EcoP15 I, Eco57M I, Fok I, Hga I, Hph I, Mbo II, Mnl I, SfaN I, TspDT I, TspDW I or Taq II.

10. The adaptor element according to claim 8 , wherein the barcode sequence is 5′-TGTCATAAAT-3′ (SEQ ID NO: 8).