The SAL stimulon
MarR and its relatives MprA and Ec17kd are transcriptional repressors acting at specific promoters. In the presence of SAL, the repressing activity of these proteins is antagonized, allowing transcription of the regulated operons. For the MarR and MprA pathways, the consequences of derepression are shown at the right. micF encodes an antisense mRNA that acts to repress production of the major outer membrane porin OmpF, and is induced in mar mutants (3). inaA is a weak acid-inducible gene whose expression is stimulated by MarA (41). For Ec17kd, the target(s) of repression have not been identified; the question mark next to the arrow leading from SAL to 17kd indicates that antagonism of 17kd-mediated repression of a normal target promoter has not been demonstrated. Other MarR homologs are predicted to carry out similar activities in controlling the expression of specific target genes.