The Bacterial Enhancer-binding Protein NtrC as a Molecular Machine (original) (raw)

1. I. ROMBEL,
2. A. NORTH,
3. I. HWANG,
4. C. WYMAN, and
5. S. KUSTU
6. Department of Plant and Microbial Biology, University of California, Berkeley, California 94720

Excerpt

The bacterial enhancer-binding protein NtrC (nitrogenregulatory protein) C functions as a molecular machine toactivate transcription by the σ54-holoenzyme form of RNApolymerase (Weiss et al. 1992; Wedel and Kustu 1995).When phosphorylated on aspartate 54, a reaction that isself-catalyzed, NtrC forms large oligomers that are capableof hydrolyzing ATP and activating transcription (Fig. 1)(Wyman et al. 1997). NtrC contacts σ54-holoenzyme fromdistant enhancer sites by means of DNA loop formation—although contact appears to be transient—and catalyzes theisomerization of closed promoter complexes to transcriptionally productive open complexes. Although σ54-holoenzyme can bind to promoters in physically detectable closedcomplexes, it cannot form open complexes in the absenceof an activator or energy source because open complexformation is thermodynamically as well as kinetically unfavorable (Wedel and Kustu 1995). Thus, NtrC is viewedas a molecular machine because it must couple the energyavailable from ATP hydrolysis to the formation of opencomplexes by σ54-holoenzyme...