The promoter is the key in both prokaryotic and eukryotic cells, and those promoters share many similar or identical features which the RNA polymerase recognizes.
Work Step by Step
See. pp. 340-341 for information on this and relevant diagrams. The promoter is a book cover in that the cover tells you that it is the start of a book you're looking for and the promoter tells RNA polymerase the same thing about a gene.