A segment of Escherichia coli bacteriophage 21 DNA encoding the late-gene regulator, Q21, and the late-gene leader RNA segment was sequenced; its structure is similar to those of the related phages lambda and 82. The leader RNA is about 45 nucleotides long and consists essentially entirely of sequences encoding the p-independent terminator that is the putative target of the antitermination activity of Q21. Like the corresponding regions of lambda and 82, the 21 late-gene promoter segment encodes an early transcription pause in vitro, at about nucleotide 18, during which Q21 presumably acts to modify RNA polymerase. The 21 Q gene, cloned in isolation, is active on the late-gene leader segment in trans, and its purified product is active as an antiterminator in vitro; Q21 represents a third late-gene antiterminator, in addition to those of lambda and 82. There is little evident similarity in the primary sequences of the three Q genes
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