REGULATION OF THE ELONGATION-TERMINATION DECISION AT INTRINSIC TERMINATORS BY ANTITERMINATION PROTEIN-N OF PHAGE-LAMBDA

The mechanisms that control N-protein-dependent antitermination in the phage lambda life cycle have counterparts in the regulatory systems o f other organisms. Here we examine N-dependent antitermination at the intrinsic tR' terminator of lambda to elucidate the regulatory princip les involved. The tR' terminator consists of a sequence of six base-pa irs along the template at which the transcription complex is sufficien tly destabilized to make RNA release possible. Within this ''zone of o pportunity'' for termination the termination efficiency (TE) at each t emplate position is determined by a kinetic competition between altern ative reaction pathways that lead either to elongation or to terminati on. TE values at each position within tR' have been mapped as a functi on of NTP concentration, and it is shown that N protein (in the presen ce of NusA and a nut site; the minimal system for N-dependent antiterm ination) can offset increases in TE that are induced by Limiting the c oncentrations of each of the next required NTPs. By limiting Nm concen trations or working at low temperature we show that a significant effe ct of N within the minimal system is to increase the rate of transcrip t elongation three- to fivefold at most positions along the template. Assuming that a comparable increase in elongation rate applies at temp late positions within the terminator, we show that an increase of this magnitude is not sufficient to account for the antitermination effici ency observed and that an similar to 100-fold stabilization of the tra nscription complex at intrinsic termination sites as a consequence of binding the N-containing antitermination sub-assembly must be invoked as well. A general method for partitioning TE effects in antiterminati on between changes in elongation rate and termination complex stabilit y is demonstrated, based on competing free energy of activation barrie rs for the elongation and termination reactions. The analysis and util ity of such mixed modes of transcriptional regulation are considered i n general terms. (C) 1997 Academic Press Limited.