Molecular Genetics

Protein factors can prevent transcription termination as well as promote it.

Facts

• The E. coli rrn operons code for the ribosomal RNA precursor. The rRNA and ribosomal protein syntheses may be coordinated for efficiency.
• A DNA site near the 5' end of the transcription unit called boxA, is the target for binding a complex of two proteins, nusB and the ribosomal protein S10.
• When either of these proteins is not present, synthesis terminates prematurely at a r -dependent terminator. When both proteins are present, the whole rrn gene is transcribed.
• Control experiments assure that translation (involved in attenuation of the trp operon) is not involved in this anti-termination.

Interpretations

• The dependence of rRNA synthesis on the presence of S10 protein not incorporated into ribosomes should contribute to balancing the synthesis of RNA and protein components of ribosomes.

Further information

• Further evidence suggests that anti-termination functions by the transfer of the boxA bound proteins to the RNA polymerase as it passes boxA. The polymerase with the nusB-S10 complex bound has altered properties and is no longer able to recognize the attenuating terminator.
• Lambda bacteriophage DNA transcription also uses anti-termination to divide the synthetic phase of the lytic cycle into three phases: immediate early, delayed early and late genes. In this case, a box A and box B sequence are involved as are additional protein factors, such as nus A and lambda-encoded proteins.

This is page 2234 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2000