Methods, systems, and reagents for direct RNA sequencing
Provided are compositions comprising recombinant polymerases that include amino acid substitutions, insertions, deletions, and/or exogenous features that confer modified properties upon the polymerase for sequencing RNA or RNA/DNA templates. Polymerases that topologically encircle the template nucleic acid are provided. Also provided are methods of using such polymerases to make a DNA or to sequence a template comprising RNA.
1. A reaction mixture comprising a polymerase enzyme complex comprising a polymerase enzyme and a template nucleic acid that comprises RNA,
wherein the polymerase is a recombinant reverse transcriptase, which recombinant reverse transcriptase comprises an amino acid sequence that is at least 80% identical to SEQ ID NO:3, and which recombinant reverse transcriptase comprises a first cysteine residue at a position selected from W25, K14, and K23 and a second cysteine residue at a position selected from K288 and A289, wherein identification of positions is relative to SEQ ID NO:3,
wherein the side chains of the first and second cysteine residues are covalently connected by a linker, thereby topologically encircling the template nucleic acid.
2. The reaction mixture of claim 1 , wherein the template nucleic acid is circular.
3. The reaction mixture of claim 1 , wherein the template nucleic acid is an mRNA.
4. The reaction mixture of claim 1 , comprising a primer hybridized to the template nucleic acid.
5. The reaction mixture of claim 1 , wherein the polymerase enzyme complex is immobilized on a surface.
6. The reaction mixture of claim 5 , comprising sequencing reagents in contact with the surface, which sequencing reagents comprise reagents for carrying out nucleic acid synthesis including one or more labeled nucleotide analogs.
7. The reaction mixture of claim 1 , comprising a phosphate-labeled nucleotide analog, wherein the polymerase incorporates the nucleotide analog into a copy nucleic acid in response to the template nucleic acid.
8. A method for sequencing a nucleic acid template comprising:
providing a polymerase enzyme complex comprising a polymerase enzyme, a template nucleic acid that comprises RNA, and optionally a primer hybridized to the template nucleic acid, wherein the polymerase is a recombinant reverse transcriptase, which recombinant reverse transcriptase comprises an amino acid sequence that is at least 80% identical to SEQ ID NO:3, and which recombinant reverse transcriptase comprises a first cysteine residue at a position selected from W25, K14, and K23 and a second cysteine residue at a position selected from K288 and A289, wherein identification of positions is relative to SEQ ID NO:3, wherein the side chains of the first and second cysteine residues are covalently connected by a linker, thereby topologically encircling the template nucleic acid, wherein the polymerase enzyme complex is immobilized on a surface;
adding sequencing reagents in contact with the surface comprising reagents for carrying out nucleic acid synthesis including one or more labeled nucleotide analogs; and
determining the sequential addition of nucleotide residues to a nucleic acid strand complementary to a strand of the template nucleic acid by observing the interaction of the labeled nucleotide analogs with the polymerase enzyme complex.
9. A system for sequencing nucleic acids comprising:
a chip comprising a plurality of polymerase enzyme complexes bound thereto, each polymerase enzyme complex individually optically resolvable, each polymerase enzyme complex comprising a polymerase enzyme, a template nucleic acid comprising RNA, and optionally a primer hybridized to the template nucleic acid, wherein the polymerase is a recombinant reverse transcriptase, which recombinant reverse transcriptase comprises an amino acid sequence that is at least 80% identical to SEQ ID NO:3, and which recombinant reverse transcriptase comprises a first cysteine residue at a position selected from W25, K14, and K23 and a second cysteine residue at a position selected from K288 and A289, wherein identification of positions is relative to SEQ ID NO:3, wherein the side chains of the first and second cysteine residues are covalently connected by a linker, thereby topologically encircling the template nucleic acid;
sequencing reagents in contact with the surface comprising reagents for carrying out nucleic acid synthesis including one or more labeled nucleotide analogs;
an illumination system for illuminating the polymerase enzyme complexes;
an optical detection system for detecting fluorescence from the labeled nucleotide analogs while they are interacting with the polymerase enzyme complexes; and
a computer for analyzing the signals detected by the detection system to determine the sequential addition of nucleotide residues to a nucleic acid strand complementary to a strand of the template nucleic acid.
10. The reaction mixture of claim 1 , wherein the recombinant reverse transcriptase comprises a first cysteine residue at position W25 and a second cysteine residue at position K288, wherein identification of positions is relative to SEQ ID NO:3, wherein the side chains of the cysteine residues at positions W25 and K288 are covalently connected by a linker.
11. The reaction mixture of claim 1 , wherein the recombinant reverse transcriptase comprises an amino acid sequence that is at least 95% identical to SEQ ID NO:3.
12. The reaction mixture of claim 1 , wherein the recombinant reverse transcriptase comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:3.