Visualization of a mammalian transcription initiation complex.

Various proteins required for the initiation of eukaryotic gene transcription by RNA polymerase II have been identified and characterized, but little is known about their organization into a functional unit. Here, we describe the appearance of the murine ribosomal protein (rp) L32 gene transcription initiation complex as determined by transmission electron microscopy. Using a fractionated nuclear extract enriched for transcription factors necessary for rpL32 gene transcription in vitro and a DNA fragment containing the rpL32 gene promoter, the transcription initiation complex was imaged by standard transmission electron microscopy. Quantitative image analysis demonstrated that the complex is a multilobed structure whose two-dimensional projections are approximately 24 x 34 nm in size. Looping of the DNA seen in these images suggests that the proteins residing at the promoter region associate with proteins several hundred base pairs distant to the RNA start site, with bending of the DNA allowing these interactions to occur.