Bacterial RNA polymerase is the first point of gene expression and a validated target for antibiotics. Studied for several decades, the Escherichia coli transcriptional apparatus is by far the best characterized, with numerous RNA polymerase mutants and auxiliary factors isolated and analyzed in great detail. Since the E. coli enzyme was refractory to crystallization, structural studies have been focused on Thermus RNA polymerases, revealing atomic details of the catalytic center and RNA polymerase interactions with nucleic acids, antibiotics, and regulatory proteins. However, numerous differences between these enzymes, including resistance of Thermus RNA polymerases to some antibiotics, underscored the importance of the E. coli enzyme structures. Three groups published these long awaited structures in 2013, enabling functional and structural studies of the same model system. This progress was made possible, in large part, by the use of multicistronic vectors for expression of the E. coli enzyme in large quantities and in a highly active form. Here we describe the commonly used vectors and procedures for purification of the E. coli RNA polymerase.