Physical evidence for the temporal transition of transcription in bacteriophage lambda

A high proportion of intracellular lambda DNA molecules are found to have D-loops, when isolated under four different conditions: (1) lambda Ots after 7 min at 31 degrees C in the presence of chloramphenicol; (2) lambda Ots after 7 min at 31 degrees C without chloramphenicol; (3) lambda Ots after 30 min at 42 degrees C; and (4) lambda cIIcIII after 50 min at 37 degrees C. The great majority of these D-loops contain RNA and are produced by E. coli RNA polymerase. In the presence of chloramphenicol, D-loops are mostly limited to the immediate early regions of the major leftward and rightward operons. At early times, with no chloramphenicol present, D-loops map primarily within the delayed early regions of the two major operons. At late times, D-loops are found mostly within the major late operon of the bacteriophage DNA. This physical evidence corroborates evidence of the temporal transition in lambda transcription obtained by other means. Chloramphenicol is shown to block the transition from immediate early to delayed early transcription.