Molecular Genetics

Gene expression in procaryotes is, in some cases, regulated by protein synthesis at the level of termination of transcription.

Facts

• In the presence of tryptophan, E. coli makes an RNA corresponding to about the 5' most 140 nt of trp operon mRNA (ref).
• In the absence of tryptophan, a full length RNA is made.
• The 3' end of the short RNA (stem loop 3-4) has characteristics of a rho-independent terminator.
• The 5' section of the stem (section 3) is also complementary to a section further 5' of it (section 2)
• The short RNA has a short protein coding region. It has 2 UGG codons in tandem in the first section.
• Stalled ribosomes cover an area of the leader RNA not including the 2-3 stem loop.
• Translating ribosomes interfere with 2-3 pairing, enabling 3-4 pairing

Interpretations

• The premature termination of transcription, called attenuation, occurs under conditions where transcription of the whole gene is not needed since there is plenty of tryptophan around for protein synthesis. Deficiency in tryptophan leads to the synthesis of the enzymes that can make more tryptophan.
• Attenuation can be responsible for the polarity of mutations in multi-cistronic operons. A nonsense mutation in an early cistron should allow stem-loops to form that do not normally form. One or more of these may cause premature termination of transcription.

Further information

• Attenuation of transcription is a frequent mechanism of regulation in E. coli.
• Attenuation is but one mechanism of regulation of the trp operon.
• In Bacillus subtilis sensing of tryptophan is done by a protein, TRAP.
• Anti-termination is another mechanism of regulation acting at transcription termination.

This is page 2233 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2000-1