Huisman O, D'Ari R, Casaregola S
Biochimie. 1982 Aug-Sep;64(8-9):709-12. doi: 10.1016/s0300-9084(82)80115-6.
The recA and sfiA genes of Escherichia coli are SOS operons regulated negatively by the LexA repressor. The steady state level of expression of recA is 10-fold higher than that of sfiA, as measured by means of recA::lac and sfiA::lac operon fusions. To study the molecular basis of this difference, we have compared the expression of these two operons in strains in which the concentration of LexA repressor was normal (lexA+), zero (spr amber mutation) or higher than normal (plasmid pJL45, carrying the lexA gene linked to the lac promoter). The results indicate (i) that the recA promoter is about 4 times stronger than the sfiA promoter (as measured in the spr strains), (ii) that neither operon has a physiologically significant level of lexA-independent expression (pJL45 strains), and (iii) that the recA operator has about 2.5 times lower affinity than the sfiA operator for LexA repressor (comparison of lex+ and spr strains). Considering our previous results that the sfiA operon (high operator affinity of LexA) is derepressed very rapidly after inducing treatments and that the recA operon (low operator affinity) is repressed very rapidly when induction is stopped, we conclude that differences in operator affinity do not affect inducibility but serve only to set the basal levels of the different SOS functions.
大肠杆菌的recA和sfiA基因是由LexA阻遏物负调控的SOS操纵子。通过recA::lac和sfiA::lac操纵子融合技术测定，recA的稳态表达水平比sfiA高10倍。为了研究这种差异的分子基础，我们比较了这两个操纵子在LexA阻遏物浓度正常（lexA+）、为零（spr琥珀突变）或高于正常（携带与lac启动子相连的lexA基因的质粒pJL45）的菌株中的表达情况。结果表明：（i）recA启动子比sfiA启动子强约4倍（在spr菌株中测定）；（ii）两个操纵子均没有与LexA无关的具有生理学意义的表达水平（pJL45菌株）；（iii）recA操纵基因对LexA阻遏物的亲和力比sfiA操纵基因低约2.5倍（lex+和spr菌株的比较）。鉴于我们之前的结果，即sfiA操纵子（LexA的操纵基因亲和力高）在诱导处理后很快去阻遏，而recA操纵子（操纵基因亲和力低）在诱导停止时很快被阻遏，我们得出结论，操纵基因亲和力的差异不影响诱导性，而仅用于设定不同SOS功能的基础水平。
