Abstract
The immC region of bacteriophage P1 contains the c1 repressor gene and its upstream region with four c1-controlled operators and four open reading frames. A c1 inactivator gene, coi, was defined by mutations in immC that suppress the virulence of the P1virC mutation. The exact location of the coi gene was not known (Scott, J.R. (1980) Curr. Top. Microbiol. Immunol. 90, 49-65). When a variety of P1 immC fragments were inserted into an expression vector, a gene product was inducible for the open reading frame 4 only. We identify this product as the c1 inactivator protein, coi by the following criteria: (a) expression of coi from a recombinant plasmid induces the P1 prophage and inhibits lysogenization of sensitive bacteria by P1; (b) all c1-controlled operator-promoter elements tested in vivo are derepressed by coi; (c) a partially purified coi protein (apparent molecular weight = 4800) interacts with c1 repressor and inhibits its binding to the operator in vitro. Based on these results we refine a model for the regulation of those genes and elements within immC which participate in the decision of P1 to enter the lytic or lysogenic pathway.
Highlights
Pl contains the cl repressor gene and its upstream region with four cl-controlled operators and four open reading frames
When the Pl BarnHI-DdeI fragment of pTH20 or pTH22 was inserted into pJF119 in the reverse orientation, no small protein related to orfX was induced visibly under the conditions described above. These findings clearly show that orf4 and not orf3 codes for the 4.8-kDa protein and that transcription of the corresponding gene starts beyond the DdeI site within the immC region, most probably from P99d (Fig. 1)
The cl inactivator protein coi is the product of the open reading frame orf4 in the immC region
Summary
Pl contains the cl repressor gene and its upstream region with four cl-controlled operators and four open reading frames. When a variety of Pl immC fragments were inserted into an expression vector, a gene product was inducible for the open reading frame 4 only We identify this product as the cl inactivator protein, coi by the following criteria: (a) expression of coi from a recombinant plasmid induces the Pl prophage and inhibits lysogenization of sensitive bacteria by Pl; (b) all cl-controlled operator-promoter elements tested in vivo are derepressed by coi; (c) a partially purified coi protein The c4 repressor prevents the expression of a closely linked gene, ant or reb, whose product, the antirepressor, interferes with the activity of the cl repressor [9, 10] Both the c4 and ant gene are located approximately at map position 52 in the imm region [11,12,13].
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