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Bacterial σ factors compete for binding to RNA polymerase (RNAP) to control promoter selection, and in some cases interact with RNAP to regulate at least the early stages of transcript elongation. However, the effective concentration of σs in vivo, and the extent to which σ can regulate transcript elongation generally, are unknown. We report that tethering σ
70
to all RNAP molecules via genetic fusion of
rpoD
to
rpoC
(encoding σ
70
and RNAP's β′ subunit, respectively) yields viable
Escherichia coli
strains in which alternative σ-factor function is not impaired. β′::σ
70
RNAP transcribed DNA normally in vitro, but allowed σ
70
-dependent pausing at extended -10-like sequences anywhere in a transcriptional unit. Based on measurement of the effective concentration of tethered σ
70
, we conclude that the effective concentration of σ
70
in
E. coli
(i.e., its thermodynamic activity) is close to its bulk concentration. At this level, σ
70
would be a bona fide elongation factor able to direct transcriptional pausing even after its release from RNAP during promoter escape.