Attenuation of the trp operon

Attenuation of the trp operon
In E. coli, attenuation is often mediated by a
tight relation of transcription and translation.
The trp mRNA leader region can exist in two alternative
base-pair conformations. One allows
transcription, the other not. When tryptophan
ist present (1), ribosomes can synthesize the
complete leader peptide. The ribosome closely
follows the RNA polymerase transcribing the
DNA template (not shown). The ribosome has
passed region 1 and prevents complementary
regions 2 and 3 from forming a hairpin by base
pairing. Instead, part of complementary region
3 and region 4 form a stem and a loop, which
favors termination of transcription. When tryptophan
is deficient (2), the ribosome stalls at the
two UGG trp codons owing to deficiency of
tryptophanyl-tRNA. This alters the conformation
of the mRNA so that regions 2 and 3 pair.
The stem structure favoring termination is not
formed, region 4 remains single-stranded, and
transcription continues.