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

Functional expression of the reverse transcriptase αβ heterodimer from avian myeloblastosis virus in Escherichia coli

The α subunit of avian myeloblastosis virus reverse transcriptase (AMV-RT) is generated from the β-subunit by proteolysis, and the αβ heterodimer represents the active form. The codon-optimized gene was expressed in Escherichia coli, and an active αβ heterodimer was generated. The RNA amplification activity of the purified recombinant AMV-RT αβ heterodimer was similar to that of the native one.

Development of a Tet-On Inducible Expression System for the Anhydrobiotic Cell Line, Pv11

The Pv11 cell line established from an African chironomid, Polypedilum vanderplanki, is the only cell line tolerant to complete desiccation. In Pv11 cells, a constitutive expression system for Pv11 cells was previously exploited and several reporter genes were successfully expressed. Here we report the identification of an effective minimal promoter for Pv11 cells and its application to the Tet-On inducible expression system. First, using a luciferase reporter assay, we showed that a 202 bp deletion fragment derived from the constitutively active 121-promoter functions in Pv11 cells as an appropriate minimal promoter with the Tet-On inducible expression system. The AcGFP1 (Aequorea coerulescens green fluorescent protein) was also successfully expressed in Pv11 cells using the inducible system. In addition to these reporter genes, the avian myeloblastosis virus reverse transcriptase α subunit (AMV RTα), which is one of the most widely commercially available RNA-dependent DNA polymerases, was successfully expressed through the inducible expression system and its catalytic activity was verified. These results demonstrate the establishment of an inducible expression system in cells that can be preserved in the dry state and highlight a possible application to the production of large and complex proteins.

2'-fluoro-modified pyrimidines enhance affinity of RNA oligonucleotides to HIV-1 reverse transcriptase

Nucleic acid aptamers can be chemically modified to enhance function, but modifying previously selected aptamers can have nontrivial structural and functional consequences. We present a reselection strategy to evaluate the impact of several modifications on preexisting aptamer pools. RNA aptamer libraries with affinity to HIV-1 reverse transcriptase (RT) were retranscribed with 2'-F, 2'-OMe, or 2'-NH₂ pyrimidines and subjected to three additional selection cycles. RT inhibition was observed for representative aptamers from several structural families identified by high-throughput sequencing when transcribed with their corresponding modifications. Thus, reselection identified specialized subsets of aptamers that tolerated chemical modifications from unmodified preenriched libraries. Inhibition was the strongest with the 2'-F-pyrimidine (2'-FY) RNAs, as compared to inhibition by the 2'-OMeY and 2'-NH₂Y RNAs. Unexpectedly, a diverse panel of retroviral RTs were strongly inhibited by all 2'-FY-modified transcripts, including sequences that do not inhibit those RTs as unmodified RNA. The magnitude of promiscuous RT inhibition was proportional to mole fraction 2'-FY in the transcript. RT binding affinity by 2'-FY transcripts was more sensitive to salt concentration than binding by unmodified transcripts, indicating that interaction with retroviral RTs is more ionic in character for 2'-FY RNA than for unmodified 2'-OH RNA. These surprising features of 2'-FY-modified RNA may have general implications for applied aptamer technologies.

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.

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.

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.