The mycoplasma-related inhibitor of HIV-1 reverse transcriptase has a DNase activity and is present in the particle-free supernatants of contaminated cultures.

Drastic inhibition of the human immunodeficiency virus (HIV) reverse transcriptase (RT) by mycoplasma has been noted in many laboratories causing confusion in data interpretation. The mycoplasma-related inhibitor of HIV-1 RT was identified as a soluble protein in the particle-free supernatant of a contaminated culture. Gel filtration studies revealed the molecular mass of this protein to be about 70 kDa. This RT-inhibitor contained a DNase with strong activity on both linear and circular DNAs. Addition of this inhibitor after completion of reverse transcription still reduced the final outcome of the RT assay significantly, implying that the inhibitory mechanism occurred mainly by its DNase activity. Treatment of the culture with an antimycoplasma drug cured the mycoplasma contamination, removed the RT-inhibitor and abolished the DNase activity.