Figure 7.

A Working Model for Initial Transcription

(A) Productive path for initial transcription. Colored columns show translocational registers adopted by growing RNA (in black). Binding site for incoming NTP is in light purple; σ3.2 loop is shown in three putative conformations (in orange). The translocational equilibrium for RP_ITC6 is controlled by several regulatory factors that modulate the lifetime of paused states arising from a pre-translocated RP_ITC6.

(B) Abortive path for initial transcription, branching from the pre-translocated RP_ITC6 state of the productive path.

(C) Path for the formation of stable backtracked scrunched states, branching from the pre-translocated RP_ITC6 state of initial transcription during NTP starvation that limits RNA synthesis to 7 nt in length.