Comparison of the thermal stabilities of reverse transcriptases from avian myeloblastosis virus and Moloney murine leukaemia virus

Enhancing thermostability of Moloney murine leukemia virus reverse transcriptase through greedy combination of multiple mutant residues

Reverse transcription is crucial in bioengineering and biomedical fields, particularly for genome sequencing and virus diagnosis. Enhancing the thermostability of reverse transcriptase can significantly improve its efficiency and accuracy by enabling it to function at higher temperatures, thereby reducing RNA secondary structures and minimizing interference from contaminating enzymes, particularly in clinical samples. Here, using a combinatorial strategy, a variant of Moloney Murine Leukemia Virus reverse transcriptase (MMLV RT) with improved activity across a wide temperature range (30-50 °C) was identified and maintained 100% activity after incubation at 50 °C for 10 min. Eleven hot-spot residues were mutated in various combinations, and the mutant proteins were rapidly expressed in a cell-free system for reverse transcription activity testing. Variant M5, which carries five mutated residues (E47K/E280R/T284R/L413G/D631V), exhibited enhanced thermostability and activity compared to any variant with a single residue mutation. Using purified recombinant protein for precise characterization, the melting temperature (Tm) of M5 increased by 4.7 °C when assembled with a nucleotide template-primer (T/P). Consequently, the half-life of M5 at 50 °C extended to approximately 60 min, in contrast to less than 4 min for the wild type. These findings demonstrate that the epistasis of combining multiple mutant residues holds excellent potential for significantly enhancing enzyme activity, even with existing knowledge. This heat-stable MMLV RT variant M5 may potentially improve efficiency and accuracy in molecular biology research and clinical diagnostics.

Left out in the cold - inequity in infectious disease control due to cold chain disparity

Vaccines and diagnostic tools stand out as among the most influential advancements in public health, credited with averting an estimated 6 million deaths annually and substantially alleviating the burden of infectious disease. Despite this progress, the global imperative to prevent, detect, and combat infectious disease persists. Regrettably, hundreds of thousands of lives are still lost due to inadequate access to vaccines and diagnostics. A critical obstacle in accessibility lies in the requirement of reliable cold chain for their transportation and storage, a resource that remains inadequate in many regions, particularly in the developing world. Various factors, including socio-economic disparities, biological complexities, and manufacturing processes, exert significant influence on the availability and integrity of vaccines and diagnostic materials. This review aims to explore the multifaceted issue of inequality in access to disease control tools, examining the vulnerabilities of vaccines and diagnostics while also investigating recent advancements that offer promising solutions to improve thermal stability.

Expression of thermostable MMLV reverse transcriptase in Escherichia coli by directed mutation

The functionality of Moloney murine leukemia virus reverse transcriptase (MMLV RT) will increase with the improvement of its solubility and thermal stability. Introduce directed mutation at specific positions of the MMLV RT sequence and codon optimization is needed to achieve these properties. The two RT coding sequences with (rRT-K) and without directed mutations (rRT-L) were versatility optimized and expressed to analyze the ribonuclease H (RNase H) inactivity and thermostable polymerase activity. For this purpose, the five-point mutations (438-442aa) and three-point mutations (530, 568, and 659 aa) were done at the RT connection domain and RNase H active site, respectively. High expression levels of rRT-L and rRT-K were obtained in E. coli BL21(DE3) and BL21(shuffle) strains, 0.5 mM IPTG concentration at 37 °C, and 8 hours' post-induction condition. Then, recombinant enzymes were purified and verified by Ni-NTA resin and western blotting. Insilico analysis (IUpred 3.0) showed that the directed mutation in the RNase H domain caused the formation of disorder regions or instability in the RNase H domain of rRT-K compared to rRT-L. The modified RT-PCR and the RT-LAMP reactions proved the RNase H inactivity of rRT-K. In addition, increasing of thermostability of rRT-K compared to rRT-L and commercial RT was evaluated by the RT-PCR and RT-LAMP reactions. The results showed that rRT-K could successfully tolerate 60 ºC in the two methods. This study revealed that the directed mutations and the versatile sequence optimization can promise to produce thermostable commercial enzymes to decrease non-specific one-step RT-PCR and RT-LAMP products.

A Novel Thermostable and Processive Reverse Transcriptase from a Group II Intron of Anoxybacillus flavithermus

Reverse transcriptases (RTs) are a family of enzymes that synthesize DNA using an RNA template and are involved in retrovirus propagation and telomere lengthening. In vitro, RTs are widely applied in various methods, including RNA-seq, RT-PCR, and RT-LAMP. Thermostable RTs from bacterial group II introns are promising tools for biotechnology due to their higher thermostability, fidelity, and processivity compared to commonly used M-MuLV RT and its mutants. However, the diversity of group II intron-encoded RTs is still understudied. In this work, we biochemically characterized a novel RT from a thermophilic bacterium, Anoxybacillus flavithermus, which was isolated from a hot spring in New Zealand and has an optimal growth temperature of around 60 °C. The cloned RT, named Afl RT, retained approximately 40% of the specific activity after a 45 min incubation at 50 °C. The optimal pH was 8.5, the optimal temperature was between 45 and 50 °C, and Mn2+ ions were found to be an optimal cofactor. The processivity analysis with MS2 phage gRNA (3569 b) demonstrated that Afl RT elongated fully up to 36% of the template molecules. In reverse transcription and RT-qLAMP, the enzyme allowed up to 10 copies of MS2 phage genomic RNA to be detected per reaction. Thus, Afl RT holds great potential for a variety of practical applications that require the use of thermostable and processive RTs.

Enzyme-Assisted Nucleic Acid Amplification in Molecular Diagnosis: A Review

Infectious diseases and tumors have become the biggest medical challenges in the 21st century. They are driven by multiple factors such as population growth, aging, climate change, genetic predispositions and more. Nucleic acid amplification technologies (NAATs) are used for rapid and accurate diagnostic testing, providing critical information in order to facilitate better follow-up treatment and prognosis. NAATs are widely used due their high sensitivity, specificity, rapid amplification and detection. It should be noted that different NAATs can be selected according to different environments and research fields; for example, isothermal amplification with a simple operation can be preferred in developing countries or resource-poor areas. In the field of translational medicine, CRISPR has shown great prospects. The core component of NAAT lies in the activity of different enzymes. As the most critical material of nucleic acid amplification, the key role of the enzyme is self-evident, playing the upmost important role in molecular diagnosis. In this review, several common enzymes used in NAATs are compared and described in detail. Furthermore, we summarize both the advances and common issues of NAATs in clinical application.

Generation of an Avian Myeloblastosis Virus (AMV) Reverse Transcriptase Prime Editor

Prime editing (PE) is a novel, double-strand break (DSB)-independent gene editing technology that represents an exciting avenue for the treatment of inherited retinal diseases (IRDs). Given the extensive and heterogenous nature of the 280 genes associated with IRDs, genome editing has presented countless complications. However, recent advances in genome editing technologies have identified PE to have tremendous potential, with the capability to ameliorate small deletions and insertions in addition to all twelve possible transition and transversion mutations. The current PE system is based on the fusion of the Streptococcus pyogenes Cas9 (SpCas9) nickase H840A mutant and an optimized Moloney murine leukemia virus (MMLV) reverse-transcriptase (RT) in conjunction with a PE guide RNA (pegRNA). In this study, we developed a prime editor based on the avian myeloblastosis virus (AMV)-RT and showed its applicability for the installation of the PRPH2 c.828+1G>A mutation in HEK293 cells.

Comparison of Reverse Transcriptase (RT) Activities of Various M-MuLV RTs for RT-LAMP Assays

Reverse transcriptases (RTs) are a family of enzymes synthesizing DNA using RNA as a template and serving as indispensable tools in studies related to RNA. M-MuLV RT and its analogs are the most commonly used RTs. RTs are widely applied in various diagnostics methods, including reverse-transcription loop-mediated isothermal amplification (RT-LAMP). However, the performance of different RTs in LAMP remains relatively unknown. Here, we report on the first direct comparison of various M-MuLV RTs in RT-LAMP, including enzymes with a different number of mutations and fusions with Sto7d. Several parameters were assessed, namely: optimal reaction temperature, enzyme concentration, reverse transcription time, a minimal amount of RNA template, and tolerance to inhibitors. Mutations increased the optimal reaction temperature from 55 °C to 60-65 °C. All of the RTs were suitable for RT-LAMP with RNA templates in the range of 10¹-10⁶ copies per reaction. Highly mutated enzymes were 1.5-3-fold more tolerant to whole blood, blood plasma, and guanidinium, but they were two-fold more sensitive to high concentrations of NaCl. The comparison of different RTs presented here could be helpful for selecting the optimal enzyme when developing novel LAMP-based diagnostic tests.

M-MuLV reverse transcriptase: Selected properties and improved mutants

Reverse transcriptases (RTs) are enzymes synthesizing DNA using RNA as the template and serving as the standard tools in modern biotechnology and molecular diagnostics. To date, the most commonly used reverse transcriptase is the enzyme from Moloney murine leukemia virus, M-MuLV RT. Since its discovery, M-MuLV RT has become indispensable for modern RNA studies; the range of M-MuLV RT applications is vast, from scientific tasks to clinical testing of human pathogens. This review will give a brief description of the structure, thermal stability, processivity, and fidelity, focusing on improving M-MuLV RT for practical usage.

Improvement of Moloney murine leukemia virus reverse transcriptase thermostability by introducing a disulfide bridge in the ribonuclease H region

Moloney murine leukemia virus (MMLV) reverse transcriptase (RT) is widely used in research and clinical diagnosis. Improvement of MMLV RT thermostability has been an important topic of research for increasing the efficiency of cDNA synthesis. In this study, we attempted to increase MMLV RT thermostability by introducing a disulfide bridge in its RNase H region using site-directed mutagenesis. Five variants were designed, focusing on the distance between the two residues to be mutated into cysteine. The variants were expressed in Escherichia coli and purified. A551C/T662C was determined to be the most thermostable variant.

End-stage renal disease reduces the expression of drug-metabolizing cytochrome P450s

BACKGROUND

End-stage renal disease is an irreversible status of kidney dysfunction that reduces both renal and non-renal drug clearance. Accumulation of uremic toxins seems to modify the activities of drug-metabolizing cytochrome P450 (CYP) enzymes. The aim of the present work was to refine gene expression analysis for efficient and accurate quantification of CYP mRNAs in patients' leukocytes.

METHODS

We compared six liquid-liquid extraction reagents for RNA isolation and five reverse transcriptase kits for RNA-to-cDNA conversion, and developed quantitative polymerase chain reaction methods for duplex measurements of CYP target genes and the reference gene. The expression of CYP1A2, CYP2C9, CYP2C19 and CYP3A4 in patients with end-stage kidney disease (N = 105) and in organ donors with healthy kidney function (N = 110) was compared.

RESULTS

Regarding the RNA yield and purity, TRIzol, Trizolate and TRI reagents were equal; however, TRI reagent was the most advantageous in terms of financial cost. Reverse transcription using Maxima First Strand cDNA Synthesis kit appeared to be the most efficient with the widest range for quantification of the target transcript. The refined method with the detection of various CYPs and the reference gene in duplex PCR efficiently quantified even the low-level CYP expression. In leukocytes of patients with end-stage renal disease, all four CYPs were expressed at significantly lower level than in organ donors with normal kidney function (p < 0.0001).

CONCLUSIONS

Reduced CYP expression was a direct evidence of transcriptional down-regulation of CYP genes in patients with impaired kidney function.

The attachment of a DNA-binding Sso7d-like protein improves processivity and resistance to inhibitors of M-MuLV reverse transcriptase

Reverse transcriptases (RTs) are a standard tool in both fundamental studies and diagnostics. RTs should possess elevated temperature optimum, high thermal stability, processivity and tolerance to contaminants. Here, we constructed a set of chimeric RTs, based on the combination of the Moloney murine leukaemia virus (M-MuLV) RT and either of two DNA-binding domains: the DNA-binding domain of the DNA ligase from Pyrococcus abyssi or the DNA-binding Sto7d protein from Sulfolobus tokodaii. The processivity and efficiency of cDNA synthesis of the chimeric RT with Sto7d at the C-end are increased several fold. The attachment of Sto7d enhances the tolerance of M-MuLV RT to the most common amplification inhibitors: NaCl, urea, guanidinium chloride, formamide, components of human whole blood and human blood plasma. Thus, fusing M-MuLV RT with an additional domain results in more robust and efficient RTs.

Alteration of enzymes and their application to nucleic acid amplification (Review)

Since the discovery of polymerase chain reaction (PCR) in 1985, several methods have been developed to achieve nucleic acid amplification, and are currently used in various fields including clinical diagnosis and life science research. Thus, a wealth of information has accumulated regarding nucleic acid-related enzymes. In this review, some nucleic acid-related enzymes were selected and the recent advances in their modification along with their application to nucleic acid amplification were described. The discussion also focused on optimization of the corresponding reaction conditions. Using newly developed enzymes under well-optimized reaction conditions, the sensitivity, specificity, and fidelity of nucleic acid tests can be improved successfully.

Engineering of a thermostable viral polymerase using metagenome-derived diversity for highly sensitive and specific RT-PCR

Reverse transcription is an essential initial step in the analysis of RNA for most PCR-based amplification and detection methods. Despite advancements in these technologies, efficient conversion of RNAs that form stable secondary structures and double-stranded RNA targets remains challenging as retroviral-derived reverse transcriptases are often not sufficiently thermostable to catalyze synthesis at temperatures high enough to completely relax these structures. Here we describe the engineering and improvement of a thermostable viral family A polymerase with inherent reverse transcriptase activity for use in RT-PCR. Using the 3173 PyroPhage polymerase, previously identified from hot spring metagenomic sampling, and additional thermostable orthologs as a source of natural diversity, we used gene shuffling for library generation and screened for novel variants that retain high thermostability and display elevated reverse transcriptase activity. We then created a fusion enzyme between a high-performing variant polymerase and the 5′→3′ nuclease domain of Taq DNA polymerase that provided compatibility with probe-based detection chemistries and enabled highly sensitive detection of structured RNA targets. This technology enables a flexible single-enzyme RT-PCR system that has several advantages compared with standard heat-labile reverse transcription methods.

Decreased Km to dNTPs is an essential M-MuLV reverse transcriptase adoption required to perform efficient cDNA synthesis in One-Step RT-PCR assay

Personalized medicine and advanced diagnostic tools based on RNA analysis are focusing on fast and direct One-Step RT-PCR assays. First strand complementary DNA (cDNA) synthesized by the reverse transcriptase (RT) is exponentially amplified in the end-point or real-time PCR. Even a minor discrepancy in PCR conditions would result in big deviations during the data analysis. Thus, One-Step RT-PCR composition is typically based on the PCR buffer. In this study, we have used compartmentalized ribosome display technique for in vitro evolution of the Moloney Murine Leukemia Virus reverse transcriptase (M-MuLV RT) that would be able to perform efficient full-length cDNA synthesis in PCR buffer optimized for Thermus aquaticus DNA polymerase. The most frequent mutations found in a selected library were analyzed. Aside from the mutations, which switch off RNase H activity of RT and are beneficial for the full-length cDNA synthesis, we have identified several mutations in the active center of the enzyme (Q221R and V223A/M), which result in 4-5-fold decrease of Km for dNTPs (<0.2 mM). The selected mutations are in surprising agreement with the natural evolution process because they transformed the active center from the oncoretroviral M-MuLV RT-type to the lenitiviral enzyme-type. We believe that this was the major and essential phenotypic adjustment required to perform fast and efficient cDNA synthesis in PCR buffer at 0.2-mM concentration of each dNTP.

Generation of thermostable Moloney murine leukemia virus reverse transcriptase variants using site saturation mutagenesis library and cell-free protein expression system

We attempted to increase the thermostability of Moloney murine leukemia virus (MMLV) reverse transcriptase (RT). The eight-site saturation mutagenesis libraries corresponding to Ala70−Arg469 in the whole MMLV RT (Thr24−Leu671), in each of which 1 out of 50 amino acid residues was replaced with other amino acid residue, were constructed. Seven-hundred and sixty eight MMLV RT clones were expressed using a cell-free protein expression system, and their thermostabilities were assessed by the temperature of thermal treatment at which they retained cDNA synthesis activity. One clone D200C was selected as the most thermostable variant. The highest temperature of thermal treatment at which D200C exhibited cDNA synthesis activity was 57ºC, which was higher than for WT (53ºC). Our results suggest that a combination of site saturation mutagenesis library and cell-free protein expression system might be useful for generation of thermostable MMLV RT in a short period of time for expression and selection.

Effects of neutral salts and pH on the activity and stability of human RNase H2

Ribonuclease H (RNase H) specifically degrades the RNA of RNA/DNA hybrid. Recent study has shown that a single ribonucleotide is embedded in DNA double strand at every few thousand base pairs in human genome, and human RNase H2 is involved in its removal. Here, we examined the effects of neutral salts and pH on the activity and stability of human RNase H2. NaCl, KCl, RbCl and NaBr increased the activity to 170-390% at 10-60 mM, while LiCl, LiBr and CsCl inhibited it, suggesting that species of cation, but not anion, is responsible for the effect on activity. NaCl and KCl increased the stability by decreasing the first-order rate constant of the inactivation to 50-60% at 60-80 mM. The activity at 25-35 °C exhibited a narrow bell-shaped pH-dependence with the acidic and alkaline pKe (pKe1 and pKe2) values of 7.3 - 7.6 and 8.1 - 8.8, respectively. Enthalpy changes (ΔH°) of deprotonation were 5 ± 21 kJ mol-1 for pKe1 and 68 ± 25 kJ mol-1 for pKe2. These results suggest that the ionizable groups responsible for pKe1 may be two out of Asp34, Glu35 and Asp141 of DEDD motif, and that for pKe2 may be Lys69 of DSK motif.

Expression of moloney murine leukemia virus reverse transcriptase in a cell-free protein expression system

OBJECTIVE

To characterize Moloney murine leukemia virus (MMLV) reverse transcriptases (RTs) expressed in a cell-free system and in Escherichia coli.

RESULTS

We previously expressed MMLV RT using an E. coli expression system and generated a highly thermostable quadruple variant MM4 (E286R/E302K/L435R/D524A) by site-directed mutagenesis. In this study, we expressed the wild-type MMLV RT (WT) and MM4 using a cell-free protein expression system from insect cells. WT exhibited DNA polymerase and RNase H activities, while MM4, in which the catalytic residue for RNase H activity, Asp524 is changed into Ala, exhibited only DNA polymerase activity. MM4, when held at 60 °C for 10 min, retained DNA polymerase activity, while WT, held at 54 °C for 10 min, lost this activity. In the cDNA synthesis reaction (0.5 μl) in which WT or MM4 were exposed to various temperatures and amounts of target RNA in a microarray chip, MM4 exhibited higher thermostability than WT.

CONCLUSION

MMLV RT expressed in the cell-free system is indistinguishable from that expressed in E. coli.

Stabilization of Moloney murine leukemia virus reverse transcriptase by site-directed mutagenesis of surface residue Val433

After thermal incubation at 48 °C for 10 min, single variants of Moloney murine leukemia virus reverse transcriptase, V433R and V433K in which a surface hydrophobic residue, Val433, was mutated, retained 55% of initial reverse transcription activity, while the wild-type enzyme retained 17%. After thermal incubation at 50 °C for 10 min, multiple variants D108R/E286R/V433R and D108R/E286R/V433R/D524A, in which Val433→Arg was combined with stabilizing mutations we identified previously, Asp108→Arg and Glu286→Arg, and RNase H activity-eliminating mutation Asp524→Ala, retained 70% of initial activity, exhibiting higher stability than V433R or V433K.

Generic detection of poleroviruses using an RT-PCR assay targeting the RdRp coding sequence

In this study a two-step RT-PCR assay was developed for the generic detection of poleroviruses. The RdRp coding region was selected as the primers' target, since it differs significantly from that of other members in the family Luteoviridae and its sequence can be more informative than other regions in the viral genome. Species specific RT-PCR assays targeting the same region were also developed for the detection of the six most widespread poleroviral species (Beet mild yellowing virus, Beet western yellows virus, Cucurbit aphid-borne virus, Carrot red leaf virus, Potato leafroll virus and Turnip yellows virus) in Greece and the collection of isolates. These isolates along with other characterized ones were used for the evaluation of the generic PCR's detection range. The developed assay efficiently amplified a 593bp RdRp fragment from 46 isolates of 10 different Polerovirus species. Phylogenetic analysis using the generic PCR's amplicon sequence showed that although it cannot accurately infer evolutionary relationships within the genus it can differentiate poleroviruses at the species level. Overall, the described generic assay could be applied for the reliable detection of Polerovirus infections and, in combination with the specific PCRs, for the identification of new and uncharacterized species in the genus.

Enzymatic characterization of human immunodeficiency virus type 1 reverse transcriptase for use in cDNA synthesis

The aim of this study is to explore the advantages of using human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) in cDNA synthesis. Recombinant HIV-1 group M (HIV-1 M) RT and HIV-1 group O (HIV-1 O) RT were produced in an Escherichia coli expression system. In the incorporation of dTTP into poly(rA)-p(dT)(15) (T/P), the K (m) values for dTTP of HIV-1 M RT and HIV-1 O RT were 8 and 12 % of that of Moloney murine leukemia virus (MMLV) RT, respectively, and the K (m) values for T/P were 25 and 23 % of that of MMLV RT, respectively. Compared with MMLV RT, HIV-1 M RT and HIV-1 O RT were less susceptible to formamide, which is frequently used for cDNA synthesis with a G + C-rich RNA to improve specificity. The high substrate affinity and low susceptibility to formamide of HIV-1 RT might be advantageous for its use in cDNA synthesis.

Improving the thermal stability of avian myeloblastosis virus reverse transcriptase α-subunit by site-directed mutagenesis

Avian myeloblastosis virus reverse transcriptase (AMV RT) is a heterodimer consisting of a 63 kDa α-subunit and a 95 kDa β subunit. Moloney murine leukaemia virus reverse transcriptase (MMLV RT) is a 75 kDa monomer. These two RTs are the most extensively used for conversion of RNA to DNA. We previously developed several mutations that increase the thermostability of MMLV RT and generated a highly stable MMLV RT variant E286R/E302K/L435R/D524A by combining three of them (Glu286→Arg, Glu302→Lys, and Leu435→Arg) and the mutation to abolish RNase H activity (Asp524→Ala) [Yasukawa et al. (2010) J Biotechnol 150:299-306]. To generate a highly stable AMV RT variant, we have introduced the triple mutation of Val238→Arg, Leu388→Arg, and Asp450→Ala into AMV RT α-subunit and the resulted variant V238R/L388R/D450A, was expressed in insect cells and purified. The temperature decreasing the initial activity by 50 %, measured over 10 min, of the variant with or without template primer (T/P), poly(rA)-p(dT)(15), was 50 °C; for the wild-type AMV RT α-subunit (WT) this was 44 °C. The highest temperature at which the variant exhibited cDNA synthesis activity was 64 °C; the WT was 60 °C. A highly stable AMV RT α-subunit is therefore generated by the same mutation strategy as applied to MMLV RT and that positive charges are introduced into RT at positions that have been implicated to interact with T/P by site-directed mutagenesis.

Mutations to create thermostable reverse transcriptase with bacterial family A DNA polymerase from Thermotoga petrophila K4

Family A DNA polymerase (K4PolI) from Thermotoga petrophila K4 was obtained as a recombinant form, and the enzyme characteristics were analyzed. K4PolI showed thermostable DNA-dependent DNA polymerase activity with 3'-5' exonuclease activity but no detectable RNA-dependent DNA polymerase activity. Its tertiary structure was speculated by in silico modeling to understand the binding situation between K4PolI and template DNA. Nine amino acids in the 3'-5' exonuclease domain are predicted to be involved in DNA/RNA distinction by steric interference with the 2' hydroxy group of ribose. To allow K4PolI to accept RNA as the template, mutants were constructed focusing on the amino acids located around the 2' hydroxyl group of the bound ribose. The mutants in which Thr326, Leu329, Gln384, Phe388, Met408, or Tyr438 was replaced with Ala (designated as T326A, L329A, Q384A, F388A, M408A, or Y438A, respectively) showed RNA-dependent DNA polymerase activity. All the mutants showed reduced 3'-5' exonuclease activity, suggesting that gain of reverse transcriptase activity is correlated with loss of 3'-5' exonuclease activity. In particular, the mutants enabled direct DNA amplification in a single tube format from structured RNA that was not efficiently amplified by retroviral reverse transcriptase.

Comparison of the thermal stabilities of the αβ heterodimer and the α subunit of avian myeloblastosis virus reverse transcriptase

Avian myeloblastosis virus reverse transcriptase (AMV RT) is a heterodimer consisting of a 63-kDa α subunit and a 95-kDa β subunit. In this study, we explored the role of the interaction between the α and β subunits on AMV RT stability. The recombinant AMV RT α subunit was expressed in insect cells and purified. It exhibited lower thermal stability than the native AMV RT αβ heterodimer. Unlike the αβ heterodimer, the α subunit was not stabilized by template-primer. These results suggest that interaction between the α and β subunits is important for AMV RT stability.

Mapping in vivo protein-RNA interactions at single-nucleotide resolution from HITS-CLIP data

Mammalian RNA complexity is regulated through interactions of RNA-binding proteins (RBPs) with their target transcripts. High-throughput sequencing together with UV-crosslinking and immunoprecipitation (HITS-CLIP) is able to globally map RBP-binding footprint regions at a resolution of ~30-60 nucleotides. Here we describe a systematic way to analyze HITS-CLIP data to identify exact crosslink sites, and thereby determine protein-RNA interactions at single-nucleotide resolution. We found that reverse transcriptase used in CLIP frequently skips the crosslinked amino-acid-RNA adduct, resulting in a nucleotide deletion. Genome-wide analysis of these crosslinking-induced mutation sites (CIMS) in HITS-CLIP data for Nova and Argonaute (Ago) proteins in mouse brain tissue revealed deletions in ~8-20% of mRNA tags, which mapped to Nova and Ago binding sites on mRNA or miRNA. CIMS analysis provides a general and more precise means of mapping protein-RNA interactions than currently available methods and insight into the biochemical properties of such interactions in living tissues.

A novel and simple method for high-level production of reverse transcriptase from Moloney murine leukemia virus (MMLV-RT) in Escherichia coli

A novel protocol for producing recombinant Moloney murine leukemia virus (MMLV-RT) in Escherichia coli is reported. The optimized coding sequence for mature MMLV-RT was cloned into pET28a and over-expressed as an N-terminal His6-tagged fusion protein. An enterokinase (EK) recognition site was introduced between the His6-tag and MMLV-RT to release tag-free enzyme. Optimal expression of soluble His6-MMLV-RT was achieved by chaperone co-expression and lower temperature fermentation. The His6-tagged enzyme was first purified by Ni(2+) affinity chromatography. The bound enzyme was then eluted by EK digestion and the eluate was purified on an anion-exchange Q column to remove DNA and EK. Twenty-one milligram MMLV-RT was obtained from 1 l of bacterial culture.

Novel mutations in Moloney Murine Leukemia Virus reverse transcriptase increase thermostability through tighter binding to template-primer

In an effort to increase the thermostability of Moloney Murine Leukemia Virus reverse transcriptase (MMLV RT), we screened random and site-saturation libraries for variants that show increased resistance to thermal inactivation. We discovered five mutations E69K, E302R, W313F, L435G and N454K that collectively increase the half-life of MMLV RT at 55°C from less than 5 min to ∼30 min in the presence of template-primer. In addition, these mutations alter the thermal profile by increasing specific activity of the pentuple mutant (M5) over a broad range of cDNA synthesis temperatures (25–70°C). We further show that M5 generates higher cDNA yields and exhibits better RT–PCR performance compared to wild-type RT when used at high temperature to amplify RNA targets containing secondary structure. Finally, we demonstrate that M5 exhibits tighter binding (lower K_m) to template-primer, which likely protects against heat inactivation.

Avian myeoloblastosis virus (AMV): only one side of the coin

For many years, scientists and suppliers have refered to AMV-RT as the reverse transcriptase produced by the Avian Myelobalstosis Virus. This manuscript briefly reviews the molecular basis for biological dependence of AMV for the envelope and RT proteins that are produced by its natural helper the Myeloblastosis Associated Virus (MAV). Because the wide use of the term <<AMV RT>> obscures scientific facts, it is worthwhile to clarify this issue for the scientific community, especially for younger scientists who might not be aware of the functional relationships that exist between these two viruses.