CoachRNA polymerase synthesises RNA on a DNA template, , from the four ribonucleoside triphosphates, without a primer. It starts at a promoter, a sequence just upstream of the gene that it recognises and binds; in bacteria a subunit called (sigma) does the recognising, at two conserved stretches ten and thirty-five pairs before the start. The enzyme opens about fifteen pairs of the helix into a transcription bubble, copies the template strand (read ) into an RNA identical in sequence to the other, coding strand, and moves along at about fifty nucleotides a second, re-closing the helix behind it; the RNA peels off as it is made. It stops at a terminator: in bacteria a self-complementary sequence whose RNA folds into a hairpin that pulls the transcript free. Many polymerases can follow one another along a gene, so a transcript can be started every second.
