ABSTRACT
Three phases of transcription Gene transcription by E. coli RNA polymerase takes place in three phases: initiation, elongation and termination. During initiation, RNA polymerase recognizes a specific site on the DNA, upstream from the gene that will be transcribed, called a promoter site and binds to it. This complex of bound RNA polymerase to the double-stranded promoter is called a closed promoter complex. The DNA then unwinds locally, creating an open promoter complex, and RNA polymerase starts transcribing the single-stranded DNA template. During elongation, the RNA polymerase uses the antisense (–) strand of DNA as template and synthesizes a complementary RNA molecule using ribonucleoside 5¢-triphosphates as precursors. The RNA produced has the same sequence as the nontemplate strand, called the sense (+) strand (or coding strand) except that the RNA contains U instead of T. At different locations on the bacterial chromosome, sometimes one strand is used as template, sometimes the other, depending on which strand is the coding strand for the gene in question. The correct strand to be used as template is identified for the RNA polymerase by the presence of the promoter site. Finally, the RNA polymerase encounters a termination signal and ceases transcription, releasing the RNA transcript and dissociating from the DNA.
