The plasmid pTHF3047 carries a 2200 bp fragment containing the phage T4 β-glucosyltransferase (β) gene and part of the upstream gene cloned in pBR313 under the control of the promoter P1. In T4-infected cells the β gene may be expressed by a mechanism antagonizing Rho action. The plasmid pTHF3047 expressed about threefold higher β-glucosyltransferase activity in a strain carrying the polarity-suppressing -102 mutation than in an otherwise isogenic strain, and production of T4 α β phage was strongly stimulated. The plasmid copy number and the total T4-specific transcription was the same in the two strains. Two T4-specific transcripts from the plasmid, 600 bases and 1850 bases, were identified by Northern hybridization. Comparison with the T4 and plasmid maps suggested that both transcripts were initiated at P1, the 600 base transcript ending at the Rho-dependent terminator t42 between gene and β, and the 1850 base transcript reading through this terminator to the end of the β gene. This analysis places t42 at position 25.1 on the T4 map, and a Rho-independent β terminator at position 23.8. The three-fold higher βgt expression in the strain may be partially accounted for by Rho control of transcription. In the strain about half of the transcripts stopped at t42, while in the strain readthrough appeared slightly higher. Thus, the t42 terminator was observed also on the plasmid, but appeared considerably less effective there than in the phage DNA in infected cells.


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