Alternative Sigma Factors in the Free State Are Equilibrium Mixtures of Open and Compact Conformations

作者：Raha Paromita; Chattopadhyay Suranjana; Mukherjee Srijata; Chattopadhyay Ruchira; Roy Koushik; Roy Siddhartha^*

来源：Biochemistry, 2010, 49(45): 9809-9819.

DOI：10.1021/bi1011173

Conformational switching upon core RNA polymerase binding is an integral part of functioning of bacterial sigma factors. Here, we have studied dynamical features of two alternative sigma factors. A study of fluorescence resonance energy transfer and hydrodynamic measurements in Escherichia cull sigma(32) suggest a compact shape like those found in complex with anti-sigma factors. On the other hand, the fluorescence anisotropy of probes attached to different regions of the protein and previous hydrogen exchange measurements suggest significant internal flexibility, particularly in the C-terminal half and region 1. In a homologous sigma factor, sigma(F) of Mycobacterium tuberculosis, emission spectra and fluorescence resonance energy transfer between the single tryptophan (W112) and probes placed in different regions suggest a compact conformation for a major part of the N-terminal half encompassing region 2 and the flexible C-terminal half. Fluorescence anisotropy measurements suggest significant flexibility in the C-terminal half and region I. as well. Thus, free alternative sigma factors may be in equilibrium between two conformations: a compact one in which the promoter interacting motifs are trapped in the wrong conformation and another less abundant one with a more open and flexible conformation. Such flexibility may be important for promoter recognition and interaction with many partner proteins.

• 出版日期2010-11-16