Optimized PCR conditions minimizing the formation of chimeric DNA molecules from MPRA plasmid libraries

Abstract Background Massively parallel reporter assays (MPRAs) enable high-throughput functional evaluation of various DNA regulatory elements and their mutant variants. The assays are based on construction of highly diverse plasmid libraries containing two variable fragments, a region of interest (a sequence under study; ROI) and a barcode (BC) used to uniquely tag each ROI, which are separated by a constant spacer sequence. The sequences of BC–ROI combinations present in the libraries may be either known a priori or not. In the latter case, it is necessary to identify these combinations before performing functional experiments. Typically, this is done by PCR amplification of the BC–ROI regions with flanking primers, followed by next-generation sequencing (NGS) of the products. However, chimeric DNA molecules formed on templates with identical spacer fragment during the amplification process may substantially hamper the identification of genuine BC–ROI combinations, and as a result lower the performance of the assays. Results To identify settings that minimize formation of chimeric products we tested a number of PCR amplification parameters, such as conventional and emulsion types of PCR, one- or two-round amplification strategies, amount of DNA template, number of PCR cycles, and the duration of the extension step. Using specific MPRA libraries as templates, we found that the two-round amplification of the BC–ROI regions with a very low initial template amount, an elongated extension step, and a specific number of PCR cycles result in as low as 0.30 and 0.32% of chimeric products for emulsion and conventional PCR approaches, respectively. Conclusions We have identified PCR parameters that ensure synthesis of specific (non-chimeric) products from highly diverse MPRA plasmid libraries. In addition, we found that there is a negligible difference in performance of emulsion and conventional PCR approaches performed with the identified settings.