Method for isolating cell type-specific mRNAs

1. A method of cell type-specific isolation of actively translated mRNA, said method comprising

(a) contacting a lysate or fraction of a heterogeneous population of cells with a reagent, said heterogeneous population of cells comprising a first subpopulation of cells comprising tagged ribosomes and a second subpopulation of cells not comprising said tagged ribosomes, wherein

(i) said tagged ribosomes comprise a ribosomal protein fused to a peptide tag, said peptide tag binding specifically to said reagent, and said peptide tag not being a ribosomal protein;

(ii) expression of said tagged ribosomal protein is under the control of a regulatory sequence which is endogenous to said first subpopulation of cells and which results in expression of said tagged ribosomal protein in said first subpopulation of cells but not in said second subpopulation of cells; and

(iii) said tagged ribosomes are bound to said actively translated mRNA; and

(b) isolating said actively translated mRNA from said lysate or fraction of the heterogeneous population of cells.

2. The method of claim 1 wherein said isolating step comprises isolating said tagged ribosomes bound to said reagent and said actively translated mRNA.

3. The method of any of claim 1 or 2 wherein said isolating step comprises isolating said actively translated mRNA from said tagged ribosomes.

4. A method of analysis of gene expression in a cell, the method comprising determining a gene expression profile consisting essentially of mRNA being actively translated, the gene expression profile being determined by a method comprising

(a) providing a cell population or a lysate thereof comprising one or more tagged ribosomes, the tagged ribosomes comprising a peptide-tagged ribosomal protein;

(b) contacting said cell population or lysate or a fraction thereof with a reagent that specifically binds said tagged ribosomes; and

(c) identifying actively translated mRNA bound to said tagged ribosomes to determine the gene expression profile consisting essentially of actively translated mRNA.

5. A method of isolation of actively translated mRNA, said method comprising

(a) contacting a population of cells or a lysate thereof with a reagent, the population of cells comprising tagged ribosomes which comprise an L10a, S15 or S18 ribosomal protein fused to a peptide tag, said peptide tag binding specifically to said reagent, said tagged ribosomes being bound to said actively translated mRNA, and

(b) isolating the actively translated mRNA from said population of cells or lysate thereof.

6. A method of cell type-specific isolation of actively translated mRNA, said method comprising:

(a) identifying a subpopulation of cells characterized by selective expression of a characterizing gene under control of an endogenous regulatory sequence,

(b) introducing an expression construct comprising the endogenous regulatory sequence and a nucleic acid encoding a peptide tag fused to a ribosomal protein into the identified subpopulation of cells,

(c) expressing said tagged ribosomal protein under the control of the endogenous regulatory sequence in said subpopulation of cells to form a tagged ribosome, wherein said tagged ribosomes bind to actively translating mRNA within said subpopulation of cells,

(d) contacting a fraction or lysate from said subpopulation with a reagent that specifically binds said tagged ribosomes, and

(e) isolating said actively translated mRNA from the subpopulation of cells or lysate thereof.

7. The method of claim 1 , 4 , 5 , or 6 , wherein the peptide tag is streptavidin and the reagent specifically binds streptavidin.

8. The method of claim 1 , 4 , or 6 , wherein the ribosomal protein is S6, S15, S18, L10a, L32, or 1.37.

9. The method of claim 1 , 4 , 5 , or 6 , wherein the ribosomal protein fused to a peptide tag is encoded by a nucleic acid comprising a first nucleotide sequence encoding the ribosomal protein fused to a second nucleotide sequence encoding the peptide tag, wherein the expression of the nucleic acid is regulated by a non-ribosomal protein regulatory sequence.

10. The method of claim 1 , 4 , 5 or 6 , wherein expression of said tagged ribosomal protein is directed by one or more transcription regulatory elements, comprising one or more transcriptional enhancer sequences, one or more insulator sequences, or a combination of said regulatory elements.

11. The method of claim 1 , 4 , 5 , or 6 , further comprising contacting said tagged ribosomes with an agent capable of arresting translation.

12. The method of claim 1 , 4 , or 5 , wherein expression of said tagged ribosomal protein is under the control of a regulatory sequence which comprises a regulatory sequence of a characterizing gene.

13. The method of claim 1 , 4 , or 6 , wherein said ribosomal protein is 1.10a, S15, or S18.

14. The method of claim 1 , 4 , 5 , or 6 , wherein said peptide tag is placed at the N- or C-terminus of said ribosomal protein.

15. The method of claim 1 , 4 , 5 , or 6 , wherein placement of said peptide tag does not prevent function of said ribosomal protein.

16. The method of claim 1 , 4 , 5 , or 6 wherein the reagent is an antibody.

17. The method of claim 16 wherein the reagent is bound to a solid surface.

18. The method of claim 1 , 4 , 5 , or 6 , wherein said peptide tag is 200 or more amino acids in length.

19. The method of claim 12 wherein said tagged ribosomes are expressed in substantially similar expression pattern as the characterizing gene.

20. The method of claim 19 wherein said tagged ribosomes are expressed in at least 80% of cells shown to express the characterizing gene by in situ hybridization.

21. The method of claim 4 wherein said cell population is a heterogenous cell population comprising a first subpopulation of cells and a second subpopulation of cells, and wherein a regulatory sequence is endogenous to said first subpopulation of cells and results in expression of said tagged ribosomal protein in said first subpopulation of cells but not in said second subpopulation of cells.

22. The method of claim 1 , 2 , or 21 wherein said regulatory sequence is at least or about 100 kilobascs in length.

23. The method of claim 1 , 2 , or 21 wherein said regulatory sequence is at least or about 200 kilobases in length.

24. The method of claim 1 , 2 , or 21 wherein said method further comprises determining a gene expression profile for said first cell subpopulation of cells.

25. The method of claim 1 , 2 , or 21 wherein said regulatory sequence endogenous to the first subpopulation of cells is associated with or part of a bacterial artificial chromosome (BAC).

26. A method of analysis of gene expression in a cell, the method comprising the method of any of claim 1 , 5 or 6 and further comprising identifying said actively translated mRNA to determine a gene expression profile of said actively translated mRNA.

27. The method of claim 4 further comprising quantifying said actively translated mRNA in the determined gene expression profile.

28. The method of claim 26 further comprising quantifying said actively translated mRNA included in the determined gene expression profile.

29. The method of claim 3 wherein said regulatory sequence endogenous to the first subpopulation of cells is associated with or part of a bacterial artificial chromosome (BAC).

30. The method of claim 3 wherein said regulatory sequence is at least or about 100 kilobases in length.

31. The method of claim 3 wherein said regulatory sequence is at least or about 200 kilohases in length.

32. The method of claim 3 wherein said method further comprises determining a gene expression profile for said first cell subpopulation.

33. The method of claim 4 wherein said peptide-tagged ribosomal protein is under the control of a regulatory sequence endogenous to said cell population.

34. The method of claim 33 wherein said regulatory sequence is associated with or part of a bacterial artificial chromosome (BAC).

35. The method of claim 33 wherein said regulatory sequence is at least or about 100 kilobases in length.

36. The method of claim 33 wherein said regulatory sequence is at least or about 200 kilobases in length.

37. The method of claim 5 wherein said tagged ribosomes are expressed under the control of a regulatory sequence endogenous to said population of cells.

38. The method of claim 37 wherein said regulatory sequence is associated with or part of a bacterial artificial chromosome (BAC).

39. The method of claim 37 wherein said regulatory sequence is at least or about 100 kilobases in length.

40. The method of claim 37 wherein said regulatory sequence is at least or about 200 kilobases in length.

41. The method of claim 5 wherein said method further comprises determining a gene expression profile for said population of cells.

42. The method of claim 6 wherein said regulatory sequence endogenous to the subpopulation of cells is associated with or part of a bacterial artificial chromosome (BAC).

43. The method of claim 42 wherein said regulatory sequence is at least or about 100 kilobases in length.

44. The method of claim 42 wherein said regulatory sequence is at least or about 200 kilobases in length.

45. The method of claim 6 wherein said method further comprises determining a gene expression profile for said subpopulation of cells.