Which subunit of E. coli RNA polymerase makes transcription start at an appropriate site rather than a random one?

1. The sigma subunit takes part only in transcription initiation. 2. It confers transcriptional specificity so that synthesis begins at an appropriate initiation site. 3. Without it the core enzyme would transcribe from random sites and produce meaningless messages. 4. The two alpha subunits assemble the polymerase on the DNA, so they position the enzyme without choosing the site. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 15 "Genes and Proteins", section 15.2 Prokaryotic Transcription_