PMID: 31131408
Fang C, Li L, Shen L, Shi J, Wang S, Feng Y, Zhang Y
Structures and mechanism of transcription initiation by bacterial ECF factors.
Nucleic Acids Res. 2019 May 27. pii: 5498757. doi: 10.1093/nar/gkz470.
Bacterial RNA polymerase (RNAP) forms distinct holoenzymes with extra-cytoplasmic function (ECF) sigma factors to initiate specific gene expression programs. In this study, we report a cryo-EM structure at 4.0 A of Escherichia coli transcription initiation complex comprising sigmaE-the most-studied bacterial ECF sigma factor (Ec sigmaE-RPo), and a crystal structure at 3.1 A of Mycobacterium tuberculosis transcription initiation complex with a chimeric sigmaH/E (Mtb sigmaH/E-RPo). The structure of Ec sigmaE-RPo reveals key interactions essential for assembly of E. coli sigmaE-RNAP holoenzyme and for promoter recognition and unwinding by E. coli sigmaE. Moreover, both structures show that the non-conserved linkers (sigma2/sigma4 linker) of the two ECF sigma factors are inserted into the active-center cleft and exit through the RNA-exit channel. We performed secondary-structure prediction of 27,670 ECF sigma factors and find that their non-conserved linkers probably reach into and exit from RNAP active-center cleft in a similar manner. Further biochemical results suggest that such sigma2/sigma4 linker plays an important role in RPo formation, abortive production and promoter escape during ECF sigma factors-mediated transcription initiation.