Summary
Transcription of the E gene (the first gene) of the tryptophan operon of Escherichia coli is abruptly reduced immediately after the onset of repression. This reduction arises from two sources: the inhibition of transcription which is in progress over an operator-proximal segment of the operon and the failure of initiation of transcription at the promoter site. Regarding the former phenomenon, kinetic analysis shows that inhibition occurs within 20 seconds after repression under conditions where it takes about 8 minutes to complete the first round of transcription of the operon. The length of the transcription region that is blocked immediately by the onset of repression is approximately the operator-proximal 4 percent of the operon. The possibility is excluded that the apparent inhibition of transcription is caused by selective and quick degradation of the operator-proximal region of the trp mRNA.
These findings support the hypothesis that the repressor inhibits transcription which is in progress over the initial protion of the operator-proximal region of the operon, thereby indirectly preventing additional RNA polymerase molecules from attaching at the promoter.
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Communicated by Jun-ichi Tomizawa
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Imamoto, F. Immediate cessation of transcription of an operator-proximal segment of the tryptophan operon in Escherichia coli following repression of the operon. Molec. Gen. Genet. 106, 123–138 (1970). https://doi.org/10.1007/BF00323831
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DOI: https://doi.org/10.1007/BF00323831