A Missense Mutation intherpoCGeneAffects Chromosomal Replication Control inEscherichia coli

AnRNA polymerase mutantwith a single-base-pair change intherpoCgene affects chromosome initiation control. Themutation, whichisrecessive, isa G toA transition leading tothesubstitution ofaspartate for glycine ataminoacidresidue 1033intheRNA polymerase j'subunit. Thechromosome copy numberis increased twofold inthemutantatsemipermissive growth temperatures (39°C). Ina AoriCstrain, inwhich chromosome initiation isgoverned byanFreplicon, chromosome copynumberisnotaffected. Plasmid pBR322 copy numberisalsoincreased inthemutantat39°C.Themutation causesa more thanfivefold increased expression ofthednaAgeneat39°C. Itisconceivable thatitisthis highDnaAconcentration which causesthe highchromosome copynumberandthatthemutantRNA polymerase ,'subunit exerts itseffect byaltering the expression ofthednaAgene.However, otherfactors mustbeaffected as welltoexplain whytheRNA polymerase mutantcan grow inabalanced fashion withahighchromosome concentration. Thisisincontrast towild-type cells, whichexhibit higher origin concentrations whenDnaAprotein isoverproduced, butinwhich theoverall DNA concentration isonlymoderately affected.