Antitermination proteins

The answer is d. (Murray, pp 435-451. Scriver, pp 3-45. Sack, pp 1—40. Wilson, pp 101—120.) Bacterial DNA contains stop signals, some of which require p protein. This has been demonstrated by examining the synthesis of mRNA in the presence and absence of p protein. In the absence of p protein, longer RNA molecules are often synthesized. This would seem to indicate that mRNA length can be controlled by the cell. In addition, antiterminator proteins are needed to allow certain genes to be properly expressed. Mammalian mechanisms for transcription termination, and the likely presence of factors regulating termination, are not yet characterized. 

Cells also contain antitermination proteins, which prevent termination of transcription of rRNA or tRNA genes at the many loops of secondary structure... 

Figure 3. Antiterminator control of transcription termination. A regulatory gene, R, codes for an antiterminator protein. The control region of DNA preceding the structural genes, SG, which in Figures 1 and 2 consisted of modulator and promoter sites, has been expanded and redefined as the leader region, which begins with a promoter, P, and ends with a terminator, T. (For simplicity, initiation of transcription is assumed to occur constitutively at the promoter.)...

In an inducible system, the antiterminator protein normally is not able to interact with a modulator site (called the liberator, Lj and transcripts initiated at P terminate at T. However, under inducing conditions when the antiterminator protein interacts with the liberator site, the transcription machinery is freed from the influence of the terminator T, and the transcripts initiated at P continue on through the adjacent structural genes. In a repressive system, the antiterminator protein normally interacts with the liberator site and there is expression of the adjacent structural genes. However, under repressing conditions when the antiterminator protein no longer interacts with the liberator, transcripts initiated at the promoter, P, terminate at the terminator, T. 

Like the activity of p Itself, the activity of the N gene product is dependent on the elongation subunit of RNA polymerase specified by the E. aoti nusA gene. It might be anticipated that attenuation of transcription by a specific antiterminating protein is a general pattern that will be found to occur frequently in microbial systems. 

The Dormant Prophage State of A Is Maintained by a Phage-Encoded Repressor Events That Follow Infection of Escherichia coli by Bacteriophage A Can Lead to Lysis or Lysogeny The N Protein Is an Antiterminator That Results in Extension of Early Transcripts Another Antiterminator, the Q Protein, Is the Key to Late Transcription... 

The N Protein Is an Antiterminator that Results in Extension of Early Transcripts... 

Roberts, J. W., RNA and protein elements of E. coli and A transcription antitermination complexes. Cell 72 653-656, 1993. 

Figure 1 Generalized model for sensing regulatory effectors by nascent mRNA leader transcripts. Transcription attenuation mechanisms have been identified in which the nascent transcript interacts with a translating 70S ribosome, a protein, an RNA molecule or a small metabolite, (a) Binding of the effector molecule promotes transcription termination, (b) Binding of the effector molecule promotes transcription readthrough (antitermination). See text for details.

TrpR, which is a DNA binding repressor protein, regulates transcription initiation of the E. coli trpEDCBA operon. Under tryptophan limiting conditions, TrpR represses transcription initiation, whereas repression is relieved in the presence of excess tryptophan. Once transcription initiates the elongating transcription complex is subject to control by transcription attenuation (reviewed in References 5 and 6). The leader transcript can form three RNA secondary structures that are referred to as the pause hairpin, the antiterminator structure, and an intrinsic terminator hairpin. Because the antiterminator shares nucleotides in common with the terminator, their formation is mutually exclusive. The pause hairpin has two additional roles in this transcription attenuation mechanism it serves as an anti-antiterminator stmc-ture that prevents antiterminator formation, and it codes for a leader peptide. A model of the E. coli trp operon transcription attenuation mechanism is presented in Fig. 2a. 

A FIGURE 11-45 Model of antitermination compiex composed of HiVTat protein and severai ceiiuiar proteins. 

Transcription of the HIV genome by RNA polymerase II Is regulated by an antitermination mechanism that requires cooperative binding by the virus-encoded Tat protein and cyclln T to the TAR sequence near the 5 end of the HIV RNA. 

---