Sort the following events in the order that they occur during transcription initiation in Escherichia coli. Your answer would be a four-letter string composed of letters A to D only; e.g. DCAB.
(A) Abortive initiation trials
(B) ? Factor release from the RNA polymerase holoenzyme
(C) Binding of the holoenzyme to the promoter in the "closed" complex
(D) Formation of the transcription bubble