Integrated—ligand-responsive microRNAs

1. A system comprising a nucleic acid comprising:

(a) an miRNA nucleic acid domain, wherein the miRNA nucleic acid domain has a basal segment region; and

(b) an RNA sensor domain that binds to a ligand, wherein the RNA sensor domain is in the basal segment region;

wherein the basal segment region is formed by:

(1) a first region that, at the 5′ end is defined by the 5′-end of the miRNA, and, at the 3′ end is defined by

(i) in a first configuration, a 5′ Drosha cleavage site, or,

(ii) in a second configuration, a 5′ of a guide sequence region, or,

(iii) in a third configuration, a 5′ of a complement of a guide sequence region; and,

(2) a second region that, at the 3′ end is defined by the 3′-end of the miRNA, and, at the 5′ end is defined by

(i) in the first configuration, a 3′ Drosha cleavage site, or,

(ii) in the second configuration, a 3′ of the complement of the guide sequence region, or,

(iii) in the third configuration, a 3′ of the guide sequence region;

wherein said first region base pairs with said second region to form a stem.

2. The system of claim 1 , wherein binding of the ligand to the sensor domain modulates processing of the miRNA nucleic acid domain by an RNA binding protein or RNA processing enzyme.

3. The system of claim 1 , wherein a portion of the miRNA nucleic acid domain is complementary to a target RNA transcript.

4. The system of claim 1 , wherein the nucleic acid comprises more than one RNA sensor domain, and wherein the sensor domains bind to the ligand.

5. The system of claim 1 , wherein the nucleic acid further comprises a second miRNA nucleic acid domain and a second RNA sensor domain, configured to bind to a ligand, wherein the RNA sensor domain is located within the basal segment region of the second miRNA nucleic acid domain.

6. The system of claim 1 , wherein the nucleic acid has more than one RNA sensor domain, and wherein at least one sensor domain binds to the ligand, and further comprising at least one additional ligand, wherein at least one sensor domain binds to the additional ligand.

7. The system of claim 2 , wherein binding of the ligand to the sensor domain inhibits processing by the RNA processing enzyme or RNA binding protein.

8. The system of claim 2 , wherein binding of the ligand to the sensor domain enhances processing by the RNA processing enzyme or RNA binding protein.

9. The system of claim 2 , wherein the RNA processing enzyme or RNA binding protein is Drosha.

10. The system of claim 2 , wherein the RNA processing enzyme or RNA binding protein is DGCR8.

11. The system of claim 1 , wherein the ligand is endogenous to a cell.

12. The system of claim 1 , wherein the ligand is exogenous to a cell.

13. The system of claim 12 , wherein the ligand is cell permeable.

14. The system of claim 1 , wherein the ligand is selected from the group consisting of polypeptides, peptides, nucleic acids, carbohydrates, fatty acids, lipids, non-peptide hormones, and metabolic precursors or products thereof.

15. The system of claim 1 , wherein the ligand has a molecular weight less than about 2.5 kDa.

16. The system of claim 1 , wherein the ligand has a molecular weight of less than about 1 kDa.

17. The system of claim 1 , wherein the ligand is theophylline, tetracycline, phenobarbital, tamoxifen, folinic acid, vitamin B12, biotin, Rev, Tat, dopamine, p50, p65, B-catenin, SAM, SAH, TPP, vitamin B1, adenine, or guanosine.

18. The system of claim 1 , wherein the miRNA down regulates expression of a target RNA.

19. The system of claim 1 , wherein the miRNA activates expression of a target RNA.

20. A cell comprising one or more nucleic acids of claim 1 .

21. The cell of claim 20 , wherein the cell is a prokaryotic cell or a eukaryotic cell.

22. The cell of claim 20 , wherein the cell is a mammalian cell.

23. The cell of claim 20 , wherein the cell is a plant cell.

24. A method of processing an miRNA, the method comprising:

(a) providing to a cell, the system of claim 1 ; and,

(b) contacting the cell with the ligand, thereby processing the miRNA.

25. The system of claim 1 , wherein the stem comprises mismatched base pairs or a bulge.

26. The system of claim 1 , wherein said first region comprises a first part of the RNA sensor domain, and said second region comprises a second part of the RNA sensor domain.

27. The system of claim 3 , wherein the guide sequence region is at least about 50%, 60%, 70%, 80%, 90%, or 95% complementary to the target RNA transcript.

28. The system of claim 1 , wherein said 5′-end of the miRNA base pairs with said 3′-end of the miRNA in said stem.