The in vitro Synthesis of Ribosomal RNA

The ribosomal RNA cistrons comprise only 0.2–0.4 % of the bacterial genome (Yankofsky and Spiegelman, 1962; Kennell, 1968). Yet in a rapidly growing culture of E. coli, rRNA synthesis can account for up to 40 % of the instantaneous rate of total RNA synthesis (Kennell, 1968; Lazzarini and Winslow, this volume). The in vivo sensitivity of rRNA synthesis to streptovaricin (Yura and Igarashi, 1968) suggests that the core RNA polymerase is involved in all such synthesis. Thus the preferential synthesis of rRNA in vivo requires that the initiation of rRNA chains by RNA polymerase must occur very rapidly. Nevertheless, Schlief (Travers, Kamen, and Schlief, in prep.) has observed that when E. coli DNA is an in vitro template for highly purified RNA polymerase containing sigma factor no detectable rRNA is synthesized (<0.2%). This suggested that such a system lacked a specificity determinant necessary for preferential...