TABLE 1.

Properties of the RNAP-binding σ-regulators.

Name	Phylum/organism	Targeted σ	Binding site	DNA interaction	Regulated process	Mode of action	Structures/PDB code
σ-activators
RbpA	Actinobacteria/Mycobacterium tuberculosis	σA, σB	σ2, β′−clamp/RNA−exit channel	Nonspecific	•Growth •Stress response •Stationary phase	σ–RNAP assembly (chaperon); Stimulates RPo formation; Stimulates promoter escape	RbpA–σA-RPo/6C04, 5TW1, 5VI5; RbpA-σA-RNAP/6C05
Crl	γ-Proteobacteria/Escherichia coli	σS	σ2/β′−CT	No	•Stress response •Stationary phase	σ–RNAP assembly (chaperon); Stimulates RPo formation	Crl–ITC5/6KJ6; Crl–RPo/6OMF
GcrA	α–Proteobacteri/Caulobacter crescentus	σA	σ2	Methylated DNA (m6A)	•Cell cycle	Stimulates RPo formation at methylated promoters	GcrA-σA/5YIX
GrgA	Chlamydiae/Chlamydia trachomatis	σA, σ28	σ2	Nonspecific	Unknown	Activates transcription initiation	–
σ-repressors
Scc4 (CT663)	Chlamydiae/Chlamydia trachomatis	σA	σ4/β-FLAP	No	• Growth • Infection	Inhibits transcription initiation at −10/−35 promoters (likely by σ4 displacement)	–
Gp39	Deinococcus-Thermus/Thermus thermophilus Phage P23-45	σA	σ4/β-FLAP	No	•Phage transcription	Inhibits RPo formation by σ4 displacement; Stimulates elongation. Anti-terminator function	RNAP-Gp39/3WOD
AsiA	γ-Proteobacteria/Escherichia coli Phage T4	σ70	σ4/β-FLAP	Nonspecific	•Phage transcription	Inhibits host RPo formation by σ4 appropriation (σ4 displacement); Stimulates phage RPo formation	RPo-AsiA-MotA/6K4Y
P7	γ-Proteobacteria/Xanthomonas oryzae Phage Xp10	No	β′−NTD/β–FLAP/RNA–exit channel	Unknown	•Phage transcription	Inhibits RPo formation by σ4 displacement; Stimulates elongation; Anti–terminator function	P7–TEC/6J9F