Digital analysis of molecular analytes using electrical methods
Electrical detection methods are used to identify and further characterize single-molecule target analytes such as proteins and nucleic acids. A composition including a probe region and a tail region is contacted with a target analyte. The probe region specifically binds to the target analyte. The tail region is coupled to the probe region, and includes a nucleic acid template for polynucleotide synthesis. When conditions are such that polynucleotide synthesis occurs along the tail region, one hydrogen ion is released for every nucleotide that is incorporated into the tail region. A transistor such as an ISFET detects and measures changes in ion concentration, and these measurements can be used to identify the tail region and thus characterize the corresponding target analyte.
1. A method of detecting a probe bound to a target analyte,
a. contacting the probe bound to a target analyte with a reaction mixture comprising reagents and under conditions that results in synthesis of a polynucleotide strand using a tail region as a template, wherein the tail region is bound to the probe, and wherein the tail region comprises a first homopolymer region of a first length of at least 25 nucleotides, a second homopolymer region of a second length of at least 25 nucleotides and a stop base between said first homopolymer region and said second homopolymer region that is distinct from the bases within the first homopolymer region and the second homopolymer region, wherein said first homopolymer region and said second homopolymer region differ by at least 25 nucleotides in length; and
b. detecting an electrical output signal during synthesis of said polynucleotide thereby detecting said probe bound to said target analyte.
2. The method of claim 1 , wherein the ratio of the length of said first homopolymer region to said second homopolymer region is 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, or 10:1 or the ratio of the length of said second homopolymer region to said first homopolymer region is 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, or 10:1.
3. The method of claim 2 , wherein said tail comprises a 5′ end and said 5′ end of tail is bound to said probe, said stop base is 3′ to said first homopolymer region, and said second homopolymer region is 3′ to said stop base.
4. The method of claim 2 , wherein said first or second homopolymeric region is at least 100 nucleotides in length.
5. The method of claim 4 , wherein said tail comprises a 5′ end and said 5′ end of tail is bound to said probe and wherein said stop base is 3′ to said first homopolymer region and said second homopolymer region is 3′ to said stop base.
6. The method of claim 2 , wherein the first or second homopolymeric region is at least 200 nucleotides in length.
7. The method of claim 6 , wherein said tail comprises a 5′ end and said 5′ end of tail is bound to said probe and wherein said stop base is 3′ to said first homopolymer region and said second homopolymer region is 3′ to said stop base.
8. The method of claim 1 , wherein said first and second homopolymer regions comprise the same base.
9. The method of claim 1 , wherein said first and second homopolymer regions comprise different bases.
10. The method of claim 1 , wherein the stop base further comprises a plurality of nucleotides distinct from the bases within the first homopolymer region and the second homopolymer region.
11. The method of claim 1 , wherein the tail region is covalently linked to the probe.
12. The method of claim 1 , wherein the first or second homopolymer region comprises a poly(dA), poly(dT), poly(dG), poly(dC), poly(A), poly(U), poly(G), or poly(C).
13. The method of claim 1 , wherein the analyte is a polypeptide, a protein, or an oligonucleotide.
14. The method of claim 1 , wherein the analyte is a small molecule, steroid, or modified nucleic acid.
15. The method of claim 1 , wherein said probe comprises an oligonucleotide, a polypeptide, an aptamer, or an antibody.
16. The method of claim 1 , wherein said electrical output signal during synthesis is detected by an ion-sensitive field effect transistor (ISFET).
17. The method of claim 1 , further comprising detecting a plurality of distinct probes comprising distinct tail regions bound to different target analytes wherein a ratio of said electrical output signal during synthesis of said first and second homopolymer synthesis is different for each distinct probe thereby providing differential detection of each of said different target analytes.
18. The method of claim 17 , wherein said tail regions of all said distinct probes are the same length.
19. The method of claim 1 , wherein said detecting an electrical output signal during synthesis of said polynucleotide comprises:
(a) contacting said probe with reagents sufficient to cause a first extension of said first homopolymer region as a template and detecting said extension;
(b) contacting said probe with reagents sufficient to cause a second extension of said stop nucleotide as a template;
(c) contacting said probe with reagents sufficient to cause a third extension of said second homopolymer region as a template and detecting said extension; and,
(d) comparing the ratio of said first extension to said second extension.
20. The method of claim 1 , wherein said probe further comprises a third homopolymer region of a third length of at least 25 nucleotides and a second stop base between said second homopolymer region and said third homopolymer region that is distinct from the bases within said first, second, and third homopolymer regions, wherein each homopolymer region differs by at least 25 nucleotides in length from the other two homopolymer regions.