Kinetic and mechanistic analysis of the RNA polymerase II transcription reaction at the human interleukin-2 promoter

Interleukin-2 (IL-2) is a cytokine critical for the proper stimulation of T-cells during the mammalian immune response. Shortly after T-cell stimulation, transcription of the IL-2 gene is upregulated. Here, we studied the kinetic mechanism of basal transcription at the IL-2 promoter using a human in vitro RNA polymerase II transcription system. We experimentally divided the transcription reaction into discrete steps, including preinitiation complex formation, initiation, escape commitment, and promoter escape. Using pre-steady state approaches, we measured the rate at which each of these steps occurs. We found that the rate of functional preinitiation complex formation limits the overall rate of transcription at the IL-2 promoter under the conditions described here. Furthermore, we found that the recruitment of TFIIF and RNA polymerase II to a TFIID/TFIIA/TFIIB/promoter complex dictates the rate of preinitiation complex formation. The rate of synthesis of 28 nt RNA from preinitiation complexes was rapid compared to the rate of preinitiation complex formation. Moreover, we found that the synthesis of a four nucleotide RNA was necessary and sufficient to rapidly complete the escape commitment step of transcription at the IL-2 promoter. Comparative experiments with the adenovirus major late promoter revealed that, while the overall mechanism of transcription is the same at the two promoters, promoter sequence and/or architecture dictate the rate of promoter escape. We present a kinetic model for a single round of basal transcription at the IL-2 promoter that provides insight into mechanisms by which the IL-2 gene is transcriptionally regulated.