arc-Dependent Thermal Regulation andExtragenic Suppression oftheEscherichia coli Cytochrome dOperon

Ina screen forEscherichia coli geneswhoseproducts arerequired forhigh-temperature growth, we identified andcharacterized amini-TnlO insertion that allows theformation ofwild-type-size colonies at30°C butresults inmicrocolony formation at36°Candabove(Ts-phenotype). Mapping, molecular cloning, and DNA sequencing analyses showed thatthemini-TnlO insertion resides inthecydBgene, thedistal geneofthe cydABoperon(cytochrome d).TheTs-growth phenotype wasalso showntobeassociated withpreviously described cydalleles. Inaddition, allcydmutants werefoundtobeextremely sensitive tohydrogen peroxide. Northern (RNA)blotanalysis showedthatcyd-specific mRNA levels accumulate following ashift tohigh temperature. Interestingly, thisheatshockinduction ofthecydoperonwasnotaffected inanrpoHA background butwastotally absent inanarcAorarcBmutantbackground. Extragenic suppressors oftheCyd Ts-phenotype arefoundat_10-3. Twoextragenic suppressors wereshowntobenull alleles ineither arcA orarcB. Oneinterpretation ofourresults isthat intheabsence ofArcAorArcB, whicharerequired forthe repression ofthecyooperon (cytochrome o),elevated levels ofCyoareproduced, thuscompensating forthe missing cytochrome dfunction. Consistent withthis interpretation, thepresence ofthecyogeneonamulticopy plasmid suppressed theTs-andhydrogen peroxide-sensitive phenotypes ofcydmutants. Bothprokaryotic andeukaryotic cells respond toavariety ofstresses byincreasing therateofsynthesis ofagroupof proteins termed heatshockproteins (for reviews, seereferences12and16). Manyoftheidentified heatshockgene products participate ineither protein folding (the GroELand DnaKchaperones) orproteolytic processing (such asLon andClpP). Theroles oftheremaining heatshockproteins arepoorly understood. Theclassical heatshockregulon has beenshowntobeunderthecontrol ofthe&2transcription factor, theproduct oftherpoH(htpR) gene(for areview, see reference 16). A second heatshockregulon isunderthe control of&C'(-E), whoseonly twoknownmembers arethe rpoHandhtrAgenes(16, 26). Recently, o-54 hasalsobeen showntoparticipate intheheatshockinduction ofyet