Principle of FPNI-PCR
The basic principle of FPNI-PCR is outlined in Figure Figure11 and and2.2. The method centers on a series of primers encompassing sequence-specific primers (designed on regions of known DNA sequence), and fusion primers which contain an arbitrary degenerate (AD) section fused to a section of determined sequence (fusion primers). The fusion of a known adaptor of determined sequence to the 5'-end of an AD (or other short site-dependent primer) is the main characteristic of FPNI-PCR (Figure (Figure1a),1a), and it is this trait which differentiates FPNI-PCR from TAIL-PCR (Figure (Figure1b).1b). The FPNI-PCR protocol contains three key steps. In the first step, a large complex mixture of DNA reactions is prepared using 0.4 μL of a gene-specific primer (SP1) designed to the genomic region of known sequence, and 2.0 μL of a combination of nine fusion arbitrary degenerate primers (eg. FP1-9; Table Table1);1); other designed primers for FPNI-PCR are presented in Additional files 1 Tables S1-S5. The details of cycling parameters and PCR conditions used in this study are listed in Table Table2.2. This first step consists of 3-6 repeats of two high stringency cycles followed by a low stringency cycle. Theoretically, single stranded PCR products from the gene-specific primer are generated during the high stringency cycles, and double-stranded products utilizing the FP primers (FPs) are developed during the low stringency cycle. After 3-6 repeated cycles of this PCR regime, it is predicted that the intended target products are partially synthesized and are accompanied by other, nonspecific, products (Figure (Figure2).2). In the second and third steps, nested PCR is conducted using 1 uL of target-specific primers (SP2/SP3, respectively) and FP-specific primers (FSP1/FSP2, respectively). These steps are high stringency PCR using high annealing temperatures so that target products are selectively amplified. Nonspecific products are not amplified in these steps, in part due to the nested approach with different primers. In addition, the large hairpin structure employed in some nonspecific products also contributes to prevent to further amplification (suppression PCR). Thus, the nonspecific products generated from the first PCR step in FPNI-PCR are not amplified in the second and third steps, and become substantially diluted in the final mix (Figure (Figure22).