How Escherichia coli sets different basal levels in SOS operons.

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.