Quantification of the carbazole 1,9a-dioxygenase gene by real-time competitive PCR combined with co-extraction of internal standards.

The fluorogenic probe assay, competitive polymerase chain reaction (PCR) and co-extraction with internal standard cells were combined to develop a rapid, sensitive, and accurate quantification method for the copy number of a target carbazole 1,9a-dioxygenase gene (carAa) and the cell number of Pseudomonas sp. strain CA10. The internal standard DNA was modified by replacement of a 20-bp long region with one for binding a specific probe in fluorogenic PCR (TaqMan). The resultant DNA fragment was similar to the corresponding region of the intact carAa gene in terms of G+C content. When used as a competitor in the PCR reaction, the internal standard DNA was distinguishable from the target carAa gene by two specific fluorogenic probes with different fluorescence labels, and was automatically detected in a single tube using the ABI7700 sequence detection system. To minimize variations in the efficiency of cell lysis and DNA extraction between the samples, the co-extraction method was combined. A mini-transposon was used to introduce competitor DNA into the genome of other pseudomonads, and the resultant construct was used as the standard cell. After the addition of a fixed amount of the internal standard cells to soil samples, total DNA was extracted (co-extraction). Using this method, the copy number of the carAa gene and the cell number of strain CA10 in soil samples could be quantified rapidly.