One-step Overlapping PCR for Rapid Synthesis of Single-guide RNA DNA Templates for the CRISPR System

The CRISPR-Cas system has revolutionized genome editing; however, conventional methods for generating single-guide RNA (sgRNA) often involve time-consuming cloning steps or expensive commercial synthesis kits. An optimized one-step overlapping PCR strategy is presented for the rapid, cost-effective synthesis of DNA templates for in vitro sgRNA transcription. Using four partially overlapping primers spanning the T7 promoter, target-specific guide sequence, and sgRNA scaffold, full-length templates are assembled in a single PCR reaction without cloning. Systematic experimental optimization established an optimal primer ratio (AF1:AF2:AF3:Tracr-R = 50:5:1:50), minimizing non-specific byproducts while maximizing full-length product yield, as confirmed by agarose gel electrophoresis. This approach was successfully extended to generate templates for Staphylococcus aureus Cas9 (saCas9) sgRNA, demonstrating cross-system applicability beyond Streptococcus pyogenes Cas9 (SpCas9). Although direct chemical synthesis of sgRNAs offers advantages such as high purity, chemical modifications to enhance stability, and reduced off-target effects, it remains prohibitively expensive for high-throughput applications or large-scale screens that require numerous sgRNAs. In vitro cleavage assays demonstrated that guide RNAs generated using this method achieve editing efficiencies comparable to those obtained via conventional plasmid-based cloning. Furthermore, ribonucleoprotein complexes assembled with these sgRNAs and delivered into HEK293T cells via electroporation resulted in detectable indel formation at the target locus, confirming functionality in vivo. Cost analysis indicates that this method substantially reduces template preparation costs compared to commercial synthesis kits while reducing turnaround time from days to hours, thereby providing an accessible and scalable approach for laboratories engaged in genetic research.