Mammalian transcription occurs stochastically in short bursts interspersed by silent intervals showing a refractory period. However, the underlying processes and consequences on fluctuations in gene products are poorly understood. Here, we use single allele time-lapse recordings in mouse cells to identify minimal models of promoter cycles, which inform on the number and durations of rate-limiting steps responsible for refractory periods. The structure of promoter cycles is gene specific and independent of genomic location. Typically, five rate-limiting steps underlie the silent periods of endogenous promoters, while minimal synthetic promoters exhibit only one. Strikingly, endogenous or synthetic promoters with TATA boxes show simplified two-state promoter cycles. Since transcriptional bursting constrains intrinsic noise depending on the number of promoter steps, this explains why TATA box genes display increased intrinsic noise genome-wide in mammals, as revealed by single-cell RNA-seq. These findings have implications for basic transcription biology and shed light on interpreting single-cell RNA-counting experiments.

SignificanceSingle-cell approaches have shown that many mammalian genes are transcribed stochastically in bursts of specific sizes and frequencies; however, molecular mechanisms controlling these bursting parameters have remained largely undetermined. By studying transcriptional bursting of a luciferase reporter controlled by a circadian gene promoter, we found that the gene integration site mainly influenced the burst size, while the circadian time primarily modulated the burst frequency. These daily variations in burst frequency correlated with histone acetylation levels, and CRISPR-Cas9–mediated acetylation of the promoter was sufficient to change the burst frequency. Since this correlation was also observed in other genes and in several cell types, we conclude that the impact of histone acetylation on gene expression is achieved mainly through modulation of burst frequency.

Highlights d We present a metabolic model for T. gondii harmonized with experimental fitness scores d T. gondii can tolerate the loss of fatty acid biosynthesis with FA supplementation d Biosynthesis of Vitamin B6 is essential in vivo and is a potential drug target d Heme biosynthesis is essential for parasite survival