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The Effect of Base Modification on RNA Polymerase And → → → The Effect of Base Modification on RNA Polymerase #707 and Reverse Transcriptase Fidelity Jennifer L. Ong, Vladimir Potapov, Xiaoqing Fu, Nan Dai, Ivan R. Corrêa Jr., Nathan Tanner New England Biolabs, Ipswich, MA 01938, USA Introduction Ribonucleic acid (RNA) is capable of hosting a variety of chemically diverse modifications. Post-transcriptional mRNA modifications can alter gene expression or mRNA stability, and can be conserved, regulated, and implicated in various cellular, developmental and disease processes. However, few studies have addressed how base modifications affect RNA polymerase and reverse transcriptase activity and fidelity, and hence, RNA sequencing data. Here, we describe the fidelity of RNA polymerization and reverse transcription of modified ribonucleotides using a fidelity assay based on Pacific Biosciences®' Single-Molecule Real-Time (SMRT®) sequencing. Several modified bases, including methylated (m6A, m5C and m5U), hydroxymethylated (hm5U) and isomeric bases (pseudouridine (Ψ)) were examined. Methods/Results Workflow Combined T7 RNA Polymerase and RT cDNA Errors A Template B 5′...CTACTATACATGACTAACAGTCGT...3′ Template dsDNA Table 1. Total error rates for cDNA strand synthesis of canonical and noncanonical RNA 0 20 40 60 80 100 120 140 160 180 dsDNA 3′...GATGATATGTACTGATTGTCAGCA...5′ T7 RNA Polymerase AAAAAAA1 |AA|AAAAAAAAAAA|AAAAAAAAAAA Template Total error rate Percentage of total errors standard (substitution/base ×10-6) RNA synthesis 5′...CUGCUAUACAUGACUAACAGUCGU...3′ RNA m6A errors/baseIntroduction Substitution Deletion Insertion AAAAAAA|AA|AAAAAAAAAAA2 |AAAAAAAAAAA RNA ×10-6 % % % Ψ MuLV 3’...GACGACATGTACTGATTGTCAGCA...5′ First strand DNA - m5C RT first strand M AAAAAAA|AA|AAAAAAAAAAA|AAAAAAAAAAA3 M-MuLV Reverse Transcriptase and T7 RNA Polymerase DNA synthesis m5U 5’...CTGCTGTACATGACTAATAGTCGT...3′ Second strand DNA RNA 63 ± 12 78 11 11 cDNA hm5U 6 (first strand) 1 T7 RNA Polymerase error m A 149 ± 21 86 9 5 standard RT second strand 2 RT error First strand error Ψ 114 ± 23 89 6 6 II m6A DNA synthesis 5 Ψ 3 m C 81 ± 18 86 9 5 dsDNA RT error Second strand error m5C product m5U 65 ± 12 87 9 4 ProtoScript m5U PacBio library Correct incorporation Misincorporation Box indicates sequenced product 5 hm U 185 ± 23 90 6 4 hm5U preparation C Substitution errors ProtoScript II Reverse Transcriptase and T7 RNA Polymerase standard Circular RNA 56 ± 8 71 19 10 m6A amplicon Ψ 1 rA→rG 2 dT→dC 3 dC→dT m6A 152 ± 8 80 11 8 PacBio SMRT AMV m5C ATG ATG UAU UAU TGT TGT Ψ 101 ± 21 90 7 3 sequencing m5U _|_ _|_ _|_ _|_ _|_ _|_ m5C 70 ± 4 82 12 6 UAC UGC ATA ACA ACA ATA hm5U Bioinformatics analysis Bst 2.0 m5U 54 ± 2 81 15 4 standard (exp.) (obs.) (exp.) (obs.) (exp.) (obs.) Bst 3.0 hm5U 188 ± 24 87 9 5 standard AMV Reverse Transcriptase and T7 RNA Polymerase (RNAP) rA→rC rA→rU rA→rG rU→rA rU→rC rU→rG rC→rA rC→rU rC→rG rG→rA rG→rC rG→rU Measuring combined transcription and reverse transcription fidelity with PacBio® (RT) dT→dG dT→dA dT→dC dA→dT dA→dG dA→dC dG→dT dG→dA dG→dC dC→dT dC→dG dC→dA RNA 75 ± 11 87 5 8 sequencing. (A) Workflow. DNA templates are transcribed by T7 RNA polymerase with standard m6A 164 ± 11 89 5 6 and modified NTPs to produce RNA. RNA is replicated by a reverse transcriptase to produce Ψ 116 ± 22 94 4 3 First strand (cDNA) synthesis error rates and error spectrum for standard and cDNA, then the first strand is replicated by the same reverse transcriptase to produce double- m5C 81 ± 2 92 3 5 stranded DNA, which is then prepared for PacBio sequencing by ligating SMRTbell™ adaptors. (B) m5U 73 ± 5 91 5 3 modified RNA. The RNA template is synthesized by T7 RNA polymerase, and then Identical first strand errors can arise by misincorporation from either the RNA polymerase or the hm5U 192 ± 8 91 5 4 reverse transcribed by the reverse transcriptases shown in the workflow. For comparison, reverse transcriptase (error type 1 and 2 in the figure, respectively). Only first strand errors Bst 2.0 Reverse Transcriptase and T7 RNA Polymerase also shown are the first strand error rates of Bst 2.0 and 3.0 DNA polymerases, DNA confirmed in the second strand are counted. Second strand errors produce a mismatch between RNA 179 ± 105 78 16 6 polymerases which can be used to reverse transcribe RNA. Polymerase substitution the first and second strand and represent misincorporation by the reverse transcriptase on DNA Bst 3.0 Reverse Transcriptase and T7 RNA Polymerase errors are written as the equivalent RNA polymerase substitution (top substitution) or templates (error type 3 in the figure). (C) Substitution errors arising from misincorporation. RNA 181 ± 102 82 15 4 reverse transcriptase substitution (bottom substitution). Base Modifications Can Alter Polymerase Fidelity Reverse Transcriptase Fidelity O O HN O NH2 hm5U HO NH Ψ HN NH m6A N N m5U NH m5C N A Second strand error rates N O O N N O N O 22 14 10 N 10 10 DNA Polymerase Total error rate Substitution Deletion Insertion 18 12 8 8 8 -6 R 10 R R R R (errors/base ×10 ) 14 8 6 6 6 10 6 4 4 4 M-MuLV RT 84 ± 19 92% 6% 3% 6 4 2 2 2 2 ProtoScript II RT 62 ± 9 91% 6% 3% 2 0 0 0 0 AMV RT 52 ± 4 93% 5% 2% -2 -2 -2 -2 -2 Bst 2.0 62 ± 5 92% 7% 1% Relative fold change fold Relative Bst 3.0 70 ± 23 89% 8% 3% Substitution type rA→rC rA→rU rA→rG rU→rA rU→rC rU→rG rC→rA rC→rU rC→rG rG→rA rG→rC rG→rU B Mutational spectrum dT→dG dT→dA dT→dC dA→dT dA→dG dA→dC dG→dT dG→dA dG→dC dC→dT dC→dG dC→dA 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% First strand error rates of modified RNA normalized to regular RNA (Protoscript® II reverse transcriptase). Relative substitution rates of each error type M-MuLV RT for each modification were normalized to unmodified RNA, for ProtoScript II reverse transcriptase (with T7 RNA polymerase). On the y-axis, E was calculated for ProtoScript II RT each substitution type as (S - M) / S, where S is the substitution rate on unmodified RNA, and M is the substitution rate on RNA containing modified bases. An E value of 0 represents no change in fidelity compared to unmodified RNA, whereas the numerical values represent the fold-change relative to unmodified RNA. AMV RT For each non-reference error identified during cDNA synthesis, the equivalent RNA polymerase error (top pair) and reverse transcriptase error (bottom pair) that could generate the corresponding firstA strand error are identifiedmismatch. Bst 2.0 RNA 5’...(rN-1)-(rΨ)-(rN+1)...3′ Bst 3.0 AAAAAAAAAAAAAAAAAAA Sequence ContextDNA 3′...(dN Analysis-1)-(dT of)-(dN Pseudouridine+1)...5′ -induced Transcription Errors dA→dC dA→dT dA→dG dT→dA dT→dC dT→dG dT→dA dT→dT dT→dG dT→dA dT→dC dT→dT B Ψ RNA unmodified'RNA Sequence context analysis of first strand errors. Sequence logos U C U represent the identity of the bases surrounding each type of A U A misincorporation, with respect to the reference RNA, for pseudouridine- Second strand error rates, representing the error rate for reverse transcriptases or C C C containing and unmodified RNA. In each logo, bases are ordered most Bst DNA polymerases replicating DNA templates. (A) Total error rates and distribution G G frequently (top) to least frequently (bottom) observed. In this example, AU U G G U A of substitution, deletion and insertion errors. (B) Normalized mutational spectrum of T7 RNA polymerase was used to generate the RNA template, and (dN )'rA:dA$ dT $ (dN )' (dN )'dT:rT$ dT $ (dN ) second strand error rates. Polymerase substitutions are written as (expected base) → $1 +1 $1 +1 ProtoScript II reverse transcriptase was used for reverse transcription. (observed base). Summary We developed an assay to measure the fidelity of cDNA synthesis on unmodified and modified RNA. cDNA (first strand) error rates are the combined error rates of T7 RNA polymerase and the reverse transcriptase. However, by normalizing base-specific error rates of the modified base to the equivalent standard RNA base, we were able to determine which modifications had an effect on either RNA polymerase or reverse transcriptase fidelity. 5-hydroxylmethyluracil and N6-methyladenosine both increased first strand error rates compared to the equivalent unmodified RNA base, whereas 5-methylcytosine and 5-methyluracil did not significantly affect first strand error rates. Pseudouridine (an isomer of uracil) was misincorporated across thymidine by T7 RNA polymerase at a greater frequency than uracil, and sequence context analysis revealed that a dT:dΨ mispair was also preferentially preceded by dG:rCTP incorporation. Reverse transcriptase-specific error rates were identified by an analysis of second strand errors, in which errors only arising during DNA templated DNA synthesis were counted. www.NEBNext.com.
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