Summary: encounters a variety of acid conditions during both its natural and pathogenic existence. The ability of this organism to respond transcriptionally to low pH is an area of active interest but little knowledge. As part of an ongoing investigation of low-pH adaptation, 18 pH-controlled operon fusions in have been identified (15 in this study) and categorized into at least 11 different loci. They include (at 57 min), (99 min), (90-93 min), (, 45 min), , (33-36 min), (93 min), (33-36 min), (59 min), (54 min) and (63 min). All but two were induced by low pH. One of the exceptions, the iron-regulated locus, was induced at high pH. The unusual locus was induced by low pH under semiaerobic conditions but high pH under aerobic conditions. Most of the other genes were expressed best under anaerobic conditions. Many of these genes exhibited strict co-inducer requirements for small molecules to be expressed in minimal medium. These included iron for , tyrosine for and , mannose for , formate for , lysine for , and unknown components of complex medium for and . Six regulatory circuits were revealed involving at least five regulatory loci ( and ). As part of the adaptive response to low pH, will induce an acid protection system called the acid tolerance response (ATR). As has been shown for mutations, the regulatory mutation interfered with the normal induction of this system.


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