Processive antitermination was first discovered in a bacteriophage, but examples have since been found in bacterial operons. The E. coli rrn operons are regulated by an antitermination mechanism that is dependent on sites that are closely related to lambda "boxA" and located promoter proximal to the 16S and 23S structural genes in each operon. The sequences of the rrn BOXA sites are more similar to the bacteriophage consensus than is that of lambda, and they bind NusB-S10 more efficiently. Although stem-loop structures analogous to BOXB are found promoter proximal to the BOXA sites, they are not essential for antitermination. An rrn BOXA sequence confers full antitermination activity against Rho-dependent but not against intrinsic terminators. BOXA also increases the rate of transcription elongation by RNAP. Point mutations in BOXA induce premature transcription termination. rrn antitermination requires NusB in vivo, as shown by a NusB depletion experiment. NusA stimulates the elongation rate of rrn RNA chains carrying BOXA. A role for NusA is further suggested by the observation that the nusA10 (Cs) mutation inhibits both antitermination and the rate of transcription elongation in an rrn operon. The role of other Nus factors in rrn regulation in vivo is not clear. In vitro, an antitermination complex that includes NusA, NusB, S10, and NusG forms at the BOXA sequence of rrnG, but these components are not sufficient for antitermination by themselves. An additional factor or factors that can be supplied by a cellular extract are required, but their identities are unknown.