Lin28/let-7 crystal structures, purification protocols, and molecular probes suitable for screening assays and therapeutics

1. An RNA oligonucleotide comprising:

a. a nucleotide sequence of formula 5′-N 1 N 2 N 3 N 4 N 5 N 6 N 7 N 8 N 9 -3′, wherein N 2 , N 4 , N 5 and N 7 are independently a purine; N 1 , N 3 , N 6 , N 8 , and N 9 are independently a pyrimidine; and

b. a nucleotide sequence of 5′-GGAG-3′, wherein the sequence 5′-GGAG-3′ is linked to the 3′ of the sequence of formula 5′-N 1 N 2 N 3 N 4 N 5 N 6 N 7 N 8 N 9 -3′, wherein the sequence 5′-GGAG-3′ is single-stranded,

wherein there are from 0 to 25 nucleotides between the 3′ end of 5′-N 1 N 2 N 3 N 4 N 5 N 6 N 7 N 8 N 9 -3′ and 5′ end of the sequence 5′-GGAG-3′, and

wherein the RNA oligonucleotide is 18-100 nucleotides in length.

2. The oligonucleotide of claim 1 , wherein the oligonucleotide comprises a hairpin structure comprising a hairpin loop of at least 3 nucleotides and N 4 , N 5 , and N 6 are in the loop region of the hairpin.

3. The oligonucleotide of claim 2 , wherein the hairpin structure comprises a double-stranded stem of at least four nucleotide base pairs, wherein the stem is fully double-stranded.

4. The oligonucleotide of claim 3 , wherein the stem comprises at least one G-clamp:G or guanadinium-G-clamp:G base pair.

5. The oligonucleotide of claim 4 , the stem is terminated by a G:C, G:U, G-clamp:G or guanadinium-G-clamp:G base pair.

6. The oligonucleotide of claim 1 , wherein the oligonucleotide comprises at least one modification selected from the group consisting of a sugar modification, a non-phosphodiester intersugar (or internucleoside) linkage, nucleobase modification, and ligand conjugation.

7. The oligonucleotide of claim 1 , wherein the oligonucleotide comprises a fluorescent reporter.

8. The oligonucleotide of claim 1 , wherein the 5′ end of the oligonucleotide is covalently linked to the 3′ end of the oligonucleotide.

9. The oligonucleotide of claim 1 , wherein N 1 and N 3 are independently selected purines and N 7 , N 8 , and N 9 are independently selected pyrimidines.

10. The oligonucleotide of claim 1 , wherein N 2 and N 4 are guanosine and N 5 is adenosine.

11. The oligonucleotide of claim 1 , wherein N 1 and N 5 are adenosine; N 3 is adenosine or uridine; N 2 and N 4 are guanosine; and N 6 , N 7 , N 8 , and N 9 are uridine.

12. The oligonucleotide of claim 1 , wherein the oligonucleotide comprises the

(SEQ ID NO: 16)

5′-GGGCAGAGAUUUUGCCCGGAG-3′

or

(SEQ ID NO: 17)

5′-GGGGUAGUGAUUUUACCCUGGAG-3′.

13. The oligonucleotide of claim 1 , wherein N 1 , N 3 , and N 6 are uridine; N 2 , N 5 , and N 7 are adenosine; N 4 is guanosine; and N 8 and N 9 are cytosine.

14. The oligonucleotide of claim 1 , wherein the oligonucleotide comprises the sequence

(SEQ ID NO: 18)

5′-GGGGUCUAUGAUACCACCCCGGAG-3′.

15. A method for promoting miRNA processing of pri-miRNA to mature miRNA in a cell, wherein the miRNA is a let-7 family member, the method comprising contacting a cell with an isolated oligonucleotide of claim 1 .