Factors Necessary for Accurate Transcription

As described above, studies of deletion and point mutations suggest that an extensive set of sequences is important in promoter recognition. Each component in these sequences that influences the activity of a promoter in vivo must be recognized by one or more transcription factors. In addition to sequence recognition factors, other components might be required to interact with pol II to permit initiation. Obviously, modulation of activity of these factors could either enhance or suppress the rate of transcription. The specificity of such regulatory signals would depend on whether a factor was specific for only a subset of promoters. 

A comparison of the in vitro transcription of mutants of several different promoters suggests that the rate of initiation is primarily dependent upon sequences between approximately - 50 and +10 (see above). In fact, the reaction in vitro on most promoters is critically responsive to sequences in the TATA consensus region. For example, converting the TAT AAA sequence in the conalbumin promoter to T AC AAA reduced the transcription activity in vitro ten-fold (Corden et al., 1980). In contrast, promoter activity in vivo is affected by mutations in the initiation region, immediate upstream region, and enhancer region. The simplest interpretation of these results is that the in vitro reaction generally reflects only one aspect of a more complicated process in vivo. 

Transcription factors have also been fractionated from a crude extract of chick oviduct cells (Tsai et al., 1981a). Fractions from these cells reconstituted accurate initiation in a reaction containing the homologous ovalbumin gene as template. Interestingly, fractions pre- 

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