Will adding too much reverse transcriptase inhibit downstream qPCR? How many units are generally appropriate to add to a 20 µL system?

After adding 1 ul of reverse transcriptase, the Ct is later than with 0.5 ul. This may indeed be because glycerol, salts, DTT, etc. in the reverse transcriptase or buffer are carried over into the subsequent qPCR. It is recommended to first fix the cDNA input amount; for example, in a 20 ul qPCR system, cDNA should not exceed 2 ul, and it is best to dilute 5-fold first and then take 2 ul. Only by diluting the cDNA from both the 0.5 ul and 1 ul enzyme addition groups at the same time and comparing them on the same plate can you determine whether this is inhibition or a template difference. A slightly later Ct value alone may also be within the error range.

For a standard 20 µL reverse transcription reaction, 1 µg RNA generally requires only 0.5–1 µL (100–200 U) of M-MLV at 200 U/µL; more enzyme is not necessarily better. In my own practice, I use 0.5 µL for 1 µg RNA, and only increase to 1 µL for low-abundance templates or when the RNA amount exceeds 2 µg. AMV or fast thermostable enzymes have different unit definitions, so calculate according to the manufacturer's recommended unit number rather than judging by volume alone. The key is to fix the enzyme amount, use a uniform cDNA dilution factor, and then compare Ct values between samples.

Recommended fixed 20 µL system: Pre-denature 1 µg RNA at 65°C for 5 minutes, then snap-chill on ice, add enzyme and primers. For mixed primers, add Oligo dT and random hexamers first and incubate at room temperature for 10 minutes, then perform reverse transcription. For reaction temperature, use 42°C for 50 minutes with standard enzymes; heat-resistant enzymes can be used at 50-55°C. After termination, inactivate at 70°C for 15 minutes, then dilute the cDNA for qPCR. If 0.5 µL enzyme is already sufficient for amplification and the melting curve is a single peak, there is no need to increase the enzyme amount; prioritize optimization through cDNA dilution and primers.

For 1 µg RNA, a mixture of random hexamers + Oligo dT primers is common, but a high proportion of random hexamers will generate large amounts of nonspecific cDNA, potentially diluting the target template and increasing background. You can try an Oligo dT:random hexamer ratio of about 3:1, or use Oligo dT alone; if the target abundance is low but the RNA is intact, gene-specific primers can be used. Changes in primer strategy will affect the composition of the cDNA product and the background of subsequent qPCR, and are more worth optimizing than simply increasing the amount of enzyme.

If buffer inhibition is suspected, a gradient experiment can be performed: add 2 µL, 1 µL, and 0.5 µL of the same cDNA to qPCR separately, and observe whether Ct decreases as the volume decreases. If adding 2 µL gives a later Ct than 1 µL, this indicates significant inhibition, and the cDNA should subsequently be diluted or the amount added should be reduced. Alternatively, purify the reverse transcription product using a spin column before qPCR to see whether Ct improves. However, purification will result in loss of low-abundance templates, so when sample quantity is limited, priority should be given to reducing the proportion of cDNA added.