Efficient human immunodeficiency virus replication requires a fine-tuned level of transcription

Transcription represents a crucial step in the life cycle of human immunodeficiency virus (HIV) and is highly regulated. Here we show that the strength of the viral long terminal repeat (LTR) promoter is optimized for efficient replication. Artificially increasing the rate of LTR-driven transcription was strongly detrimental for viral fitness, and HIV was able to regain replication capacity by selecting for variants with a weaker LTR. Strikingly, the strength of the evolved promoter was equivalent to that of the wild-type LTR