تسجيل الدخول

تسجيل

Principles of Gene Regulation:- RNA Polymerase Binds to DNA at Promoters

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  p1082-1083

2026-07-30

27

Principles of Gene Regulation:- RNA Polymerase Binds to DNA at Promoters

RNA polymerases bind to DNA and initiate transcription at promoters , sites generally found near points at which RNA synthesis begins on the DNA template. The regulation of transcription initiation of ten entails changes in how RNA polymerase interacts with a promoter. The nucleotide sequences of promoters vary considerably, affecting the binding affinity of RNA polymerases and thus the frequency of transcription initiation. Some Escherichia coli genes are transcribed once per second, others less than once per cell generation. Much of this variation is due to differences in promoter sequence. In the absence of regulatory proteins, differences in promoter sequences may affect the frequency of transcription initiation by a factor of 1,000 or more. Most E. coli promoters have a sequence close to a consensus (Fig. 1). Mutations that result in a shift away from the consensus sequence usually decrease promoter function; conversely, mutations toward consensus usually enhance promoter function. Although housekeeping genes are expressed constitutively, the cellular concentrations of the proteins they encode vary widely. For these genes, the RNA polymerase–promoter interaction strongly influences the rate of transcription initiation; differences in promoter sequence allow the cell to synthesize the appropriate level of each housekeeping gene product. The basal rate of transcription initiation at the promoters of nonhousekeeping genes is also determined by the promoter sequence, but expression of these genes is further modulated by regulatory proteins. Many of these proteins work by enhancing or interfering with the interaction between RNA polymerase and the promoter. The sequences of eukaryotic promoters are more variable than their prokaryotic counterparts . The three eukaryotic RNA polymerases usually require an array of general transcription factors in order to bind to a promoter. Yet, as with prokaryotic gene expression, the basal level of transcription is determined by the effect of promoter sequences on the function of RNA polymerase and its associated tran scription factors.

FIGURE 1 Consensus sequence for many E. coli promoters. Most base substitutions in the 10 and 35 regions have a negative effect on promoter function. Some promoters also include the UP (upstream promoter) element . By convention, DNA sequences are shown as they exist in the nontemplate strand, with the 5 terminus on the left. Nucleotides are numbered from the transcription start site, with positive numbers to the right (in the direction of transcription) and negative numbers to the left. N indicates any nucleotide.