doukhan l predich m nair g dussurget o mandic mulec i cole s t smith d r and smith i 1995 gene 165 67 70 schmitt m p predich m doukhan l smith i and holmes r k 1995 infect immun 63 4284 4289 oguiza i a tao x marcos a t martin j f and murphy j r 1995 j bacteriol 177 465 467 gunter k toupet c and schupp t 1993 j bacteriol 175 3295 3302 these repressors have been implicated in the iron dependent gene regulation in their respective hosts of origin boyd j oza m and murphy j r 1990 proc natl acad sci usa 87 5968 5972 schmitt m p and holmes r k 1991 infect immun 59 1899 1904 schmitt m p and holmes r k 1994 j bacteriol 176 1141 1149 schmitt m p and holmes r k 1994 j bacteriol 176 1141 1149 schmitt m p 1997 j bacteriol 179 838 845 furthermore table 1 above has indicated an additional collection of bacterial species which have one or more identical copies of the toxo operator sequence suggesting they may employ an analogous metal activated repressor dependent regulatory mechanism for state specific gene expression given the potentially important role played by dtxr and dtxr homologues in pathogenesis we have developed a positive genetic selection system for the rapid isolation of functionally equivalent dtxr alleles dtxr homologues and dtxr target dna binding or u201c iron box u201d sites the psdt system as described above is used in this example to.