Editing Sigma factor (section)

== Sigma factor competition ==

The number of RNAPs in bacterial cells (e.g., ''E. coli'') have been shown to be smaller than the number of sigma factors. Consequently, if a certain sigma factor is overexpressed, not only will it increase the expression levels of genes whose promoters have preference for that sigma factor, but it will also reduce the probability that genes with promoters with preference for other sigma factors will be expressed.<ref>{{Cite journal |last1=Jishage |first1=M |last2=Ishihama |first2=A |date=December 1995 |title=Regulation of RNA polymerase sigma subunit synthesis in Escherichia coli: intracellular levels of sigma 70 and sigma 38 |journal=Journal of Bacteriology |language=en |volume=177 |issue=23 |pages=6832–6835 |doi=10.1128/jb.177.23.6832-6835.1995 |issn=0021-9193 |pmc=177550 |pmid=7592475}}</ref><ref>{{Cite journal |last1=Jishage |first1=M |last2=Iwata |first2=A |last3=Ueda |first3=S |last4=Ishihama |first4=A |date=September 1996 |title=Regulation of RNA polymerase sigma subunit synthesis in Escherichia coli: intracellular levels of four species of sigma subunit under various growth conditions |journal=Journal of Bacteriology |language=en |volume=178 |issue=18 |pages=5447–5451 |doi=10.1128/jb.178.18.5447-5451.1996 |issn=0021-9193 |pmc=178365 |pmid=8808934}}</ref><ref>{{Cite journal |last1=Grigorova |first1=Irina L. |last2=Phleger |first2=Naum J. |last3=Mutalik |first3=Vivek K. |last4=Gross |first4=Carol A. |date=2006-04-04 |title=Insights into transcriptional regulation and σ competition from an equilibrium model of RNA polymerase binding to DNA |journal=Proceedings of the National Academy of Sciences |language=en |volume=103 |issue=14 |pages=5332–5337 |doi=10.1073/pnas.0600828103 |pmid=16567622 |pmc=1459355 |bibcode=2006PNAS..103.5332G |issn=0027-8424|doi-access=free }}</ref><ref name=":0">{{Cite journal |last1=Kandavalli |first1=Vinodh K. |last2=Tran |first2=Huy |last3=Ribeiro |first3=Andre S. |date=October 2016 |title=Effects of σ factor competition are promoter initiation kinetics dependent |url=https://linkinghub.elsevier.com/retrieve/pii/S1874939916301614 |journal=Biochimica et Biophysica Acta (BBA) - Gene Regulatory Mechanisms |language=en |volume=1859 |issue=10 |pages=1281–1288 |doi=10.1016/j.bbagrm.2016.07.011|pmid=27452766 |url-access=subscription }}</ref>

Meanwhile, transcription initiation has two major rate limiting steps: the closed and the open complex formation. However, only the dynamics of the first step depends on the concentration of sigma factors. Interestingly, the faster the closed complex formation relative to the open complex formation, the less responsive is a promoter to changes in sigma factors’ concentration (see <ref name=":0" /> for a model and empirical data of this phenomenon).