2.
Summary By using an artificial hybrid between phage and the pR plasmid, we have shown that the
rep region of the pR plasmid encodes a function which regulates the expression of the
muc genes (plasmid genes that are under the negative control of
lexA and responsible for an increased rate of spontaneous mutagenesis and resistance to UV and chemicals). Expression of the
muc genes was monitored by a fusion between the
muc promoter and the
lacZ structural gene. When
E. coli cells containing such a fusion are infected by the hybrid pR phasmid, -galactosidase activity is enhanced, indicating that pR encodes an antagonist of
lexA. By deletion mapping we have located the gene encoding the antagonist of
lexA (
bat) in the
rep region of the plasmid. The
bat gene product can also antagonize the
cI repressor as shown by the observation that pR phasmids are virulent on a homoimmune lysogen. We have exploited this latter property to carry out genetic and functional analysis of the
bat region. This region is organized as a classical operon where the expression of the
bat structural gene is negatively regulated by a repressor gene that encodes a proteic product.
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