One of the most widely applied and foundational nucleic acid amplification technologies (NAATs) is the polymerase chain reaction (PCR), first conceived by Kary Mullis in the early 1980s (Mullis et al., 1986). PCR amplifies DNA through repeated cycles of denaturation, annealing, and extension using sequence-specific primers, thermostable DNA polymerase, nucleotides, and magnesium ions (Singh et al., 2014). Typically comprising 30 to 40 cycles, the reaction generates millions of amplicons and supports diverse applications, including diagnostics, forensics, gene expression analysis, cloning, sequencing, and mutation detection (Singh et al., 2014). RNA targets can also be analyzed by converting RNA into complementary DNA (cDNA) via reverse transcription (Singh et al., 2014). Despite its broad utility, PCR requires precise temperature cycling, specialized and often costly equipment, and trained personnel, which limits its suitability for rapid or on-site detection (Yan et al., 2014).
Isothermal nucleic acid amplification technologies offer a significant advancement over conventional PCR by enabling target amplification at a single, constant temperature (Yan et al., 2014). Although both PCR and isothermal methods rely on polymerases to synthesize new DNA strands, isothermal approaches bypass the need for high-temperature denaturation to unwind double-stranded DNA and separate nascent products (Yan et al., 2014). Each isothermal method employs a distinct reaction mechanism and a specialized combination of components, such as single-stranded DNA-binding proteins, helicases, nicking enzymes, recombinases, restriction enzymes, or polymerases with strand-displacement activity, to achieve amplification without thermal cycling (Yan et al., 2014; Li et al., 2018). Among these technologies, recombinase polymerase amplification (RPA) and recombinase-aided amplification (RAA) have emerged as widely adopted platforms for rapid and equipment-free nucleic acid detection (Wu et al.,2024; Xie et al.,2025).
Recombinase polymerase amplification (RPA)
RPA, developed in 2006 and later commercialized by TwistDx, has emerged as one of the most prominent isothermal nucleic acid amplification technologies (Piepenburg et al.,2006). Its principal advantage over PCR is its ability to operate at a constant low temperature (37 to 42°C), eliminating the need for thermocyclers and enabling rapid, portable, and field-deployable molecular detection (Wu et al.,2024). The reaction is typically completed within 5 to 20 minutes, reflecting its high amplification efficiency and suitability for point-of-care testing (POCT) (Wu et al.,2024).
Mechanistically, RPA relies on the coordinated activities of a recombinase (T4 UvsX), single-stranded DNA-binding protein (T4 Gp32), and a strand-displacing DNA polymerase (Bsu) (Wu et al.,2024). The recombinase first binds to the primer to form a nucleoprotein complex that searches for and invades the homologous sequence within the double-stranded template (Wu et al.,2024). Gp32 stabilizes the displaced single-stranded region, after which the recombinase dissociates, permitting Bsu polymerase to extend from the primer’s 3’ end and synthesize new DNA (Wu et al.,2024). Repetition of this process drives exponential amplification (Figure 1).
These biochemical properties contribute to RPA’s high sensitivity, operational simplicity, and tolerance to mismatches and common inhibitors. RPA is compatible with multiplexing and multiple detection formats, including gel electrophoresis, real-time fluorescence, and lateral flow assays, supporting its broad use in molecular diagnostics. Its versatility has enabled applications across numerous fields, including pathogen detection (viruses, bacteria, fungi, parasites), drug-resistance monitoring, food safety testing, and agricultural and veterinary surveillance (Wu et al.,2024).
Despite its strengths, RPA faces several technical limitations. Manual sample handling increases the likelihood of aerosol contamination, prompting the development of closed-tube strategies such as integrating RPA with CRISPR-Cas13 detection. Primer and probe design remain challenging due to the absence of universal design guidelines and dedicated software, though tools like PrimedRPA represent progress toward automated, specificity-guided primer selection. Furthermore, the lack of thermal cycling makes RPA prone to primer dimer formation and nonspecific amplification, particularly at low template concentrations. Approaches such as self-avoiding molecular recognition systems have been proposed to mitigate these issues (Wu et al.,2024).
Recombinase-aided amplification (RAA)
In the case of RAA, the technique is a rapid isothermal nucleic acid amplification technology developed in China in 2010 (Bei et al.,2010). Mechanistically, RAA closely resembles RPA, relying on the coordinated activities of a recombinase (typically of bacterial or fungal origin), a single-stranded DNA-binding protein, and a DNA polymerase (Bei et al.,2010). The reaction begins when the recombinase and single-stranded DNA-binding protein bind to the primer in the presence of ATP, forming a nucleoprotein complex that interrogates the double-stranded DNA template (Bei et al.,2010). Upon locating a complementary sequence, a stable D-loop structure is generated, after which the polymerase extends the primer using dNTPs to synthesize a new DNA strand (8). The entire process proceeds at a constant temperature, typically around 37 to 42°C, enabling exponential amplification within 15 to 30 minutes (Bei et al.,2010). Similar to RPA, RAA offers key advantages, including simplicity, speed, and suitability for non-laboratory, POCT due to its isothermal nature (Bei et al.,2010). The applications and detection formats for RAA are analogous to those of RPA (Bei et al.,2010). However, a significant technical constraint for RAA remains the absence of dedicated software tools to facilitate efficient primer design (Wu et al.,2024). A comparative summary of RPA and RAA is presented in Table 1.
| Feature | RPA | RAA |
|---|---|---|
| Reagent | recombinase (T4 UvsX), single-stranded DNA-binding protein (T4 Gp32), strand-displacing DNA polymerase (Bsu) | recombinase, single-stranded binding protein, DNA polymerase |
| Primer | 1 pair | 1 pair |
| Operating temperature | 37-42°C | 37-42°C |
| Reaction time | 5-20 min | 15-30 min |
| Template | DNA | DNA |
| Instruments | simple heater / portable device | simple heater / portable device |
| Sensitivity | 1-10 copies/reaction | 1-10 copies/reaction |
| Specificity | high | high |
| Multiplexing capacity | moderate | limited |
| POCT adaptability | high | high |
| Regulatory validation | limited | limited |
| Cost and complexity | low-moderate | low |
Table 1. Comparative analysis of recombinase polymerase amplification (RPA) and recombinase-aided amplification (RAA).
In summary, although PCR remains a foundational NAAT, its requirement for precise thermal cycling restricts its utility in field-based diagnostics. Isothermal methods such as RPA and RAA overcome this constraint by employing recombinase-mediated primer targeting and strand-displacement synthesis to achieve rapid, exponential amplification at low, constant temperatures (37 to 42°C). These features enable portable, rapid, and equipment-free detection across diverse fields. Despite their strong potential as POCT, both RPA and RAA face notable challenges, including susceptibility to aerosol contamination and the absence of dedicated, standardized primer-design software.
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Parut Suksai
The Monitoring and Surveillance Center for Zoonotic Diseases in Wildlife and Exotic Animals (MoZWE), Faculty of Veterinary Science, Mahidol University