The Use of Degenerate Primers in qPCR Analysis of Functional Genes Can Cause Dramatic Quantification Bias as Revealed by Investigation of nifH Primer Performance

The measurement of functional gene abundance in diverse microbial communities often employs quantitative PCR (qPCR) with highly degenerate oligonucleotide primers. While degenerate PCR primers have been demonstrated to cause template-specific bias in PCR applications, the effect of such bias on qPCR has been less well explored. We used a set of diverse, full-length nifH gene standards to test the performance of several universal nifH primer sets in qPCR. We found significant template-specific bias in all but the PolF/PolR primer set. Template-specific bias caused more than 1000-fold mis-estimation of nifH gene copy number for three of the primer sets and one primer set resulted in more than 10,000-fold mis-estimation. Furthermore, such template-specific bias will cause qPCR estimates to vary in response to beta-diversity, thereby causing mis-estimation of changes in gene copy number. A reduction in bias was achieved by increasing the primer concentration. We conclude that degenerate primers should be evaluated across a range of templates, annealing temperatures, and primer concentrations to evaluate the potential for template-specific bias prior to their use in qPCR.