For reverse transcription of low-copy genes, is ordinary M-MLV or thermostable reverse transcriptase more suitable?

At low template amounts, I would recommend prioritizing RNase H-deficient or thermostable M-MLV over continuing with standard M-MLV. When total RNA is only a few tens of nanograms, standard M-MLV does not sufficiently open RNA secondary structures at the conventional 42°C, and cDNA yield tends to be low; RNase H-deficient variants can reduce degradation of the RNA template during reverse transcription, usually giving better product length and yield; thermostable variants can react at 50-55°C and are more friendly to low-abundance targets. With a budget of under 500 per unit, there are many options; it is advisable to first get a small pack and compare it side by side with standard enzyme on the same plate using the same RNA, to see whether the Ct values and melting curves of low-abundance targets improve.

When total RNA is only a few dozen nanograms, a high Ct value is not necessarily all due to the enzyme. It is recommended to first take a small amount of RNA for electrophoresis or check the RIN value to confirm whether the 28S/18S bands are clear and whether there is smearing. If the RNA is already degraded or has poor integrity, switching to a thermostable enzyme may also provide limited improvement. Spectrophotometer readings tend to be falsely elevated at low concentrations, so it is best to confirm with electrophoresis or fluorescence quantification. If the RNA is indeed intact, then compare regular M-MLV and thermostable/RNase H-deficient versions using the same sample, and the results will be more reliable.

The sample volume is low, so it is not recommended to continue using the conventional 20 µL system. Consider reducing the reverse transcription volume to 10 µL or even lower to increase the final cDNA concentration. Regarding primers, random hexamers provide better coverage for low-abundance mRNA, while Oligo dT may result in lower yield due to the low template amount; gene-specific reverse transcription primers can also be designed for the target gene. Calculate the enzyme amount based on the RNA amount; for several tens of nanograms, 100–200 U is generally sufficient. Use low-adsorption tubes during operation to reduce template loss from adsorption.

For budgets under 500, focus on RNase H-deficient M-MLV, thermostable, or premixed kits. When contacting suppliers, ask clearly about enzyme activity units, whether RNase inhibitors are included, and whether trial samples are available. The key experimental design is to split the same RNA into multiple tubes and perform reverse transcription in parallel with standard M-MLV, thermostable, and RNase H-deficient enzymes, followed by qPCR on the same plate, focusing on comparing Ct differences for low-abundance targets, amplification efficiency, and melting curves. Do not compare across different RNA batches, otherwise it is difficult to determine whether the improvement comes from the enzyme or the sample.

If the background is dirty, first rule out gDNA contamination or primer dimers rather than simply trying a different enzyme. It is recommended to set up a no-RT control and an NTC control. If the no-RT control also shows a signal, it indicates residual gDNA from extraction, and DNase I treatment followed by purification should be performed, or the qPCR primers should be designed to span an intron. Thermostable and RNase H-deficient types mainly improve cDNA yield and specificity, but they will not directly eliminate gDNA contamination. First make the controls rigorous, then determine whether it is worth switching enzymes; otherwise, you may be testing in vain.