Mutual suppression of mukB and seqA phenotypes might arise from their opposing influences on the Escherichia coli nucleoid structure

A strain of Escherichia coli in which both the seqA and mukB genes were ina ctivated displayed partial suppressions of their individual phenotypes. Tem perature sensitivity, anucleate cell production and poor nucleoid folding s een in the mukB strain were suppressed by the seqA null mutation, whereas f ilamentation, asymmetric septation and compact folding of the nucleoids obs erved in the seqA strain were suppressed by inactivation of the mukB gene f unction, However, the asynchronous initiation of chromosome replication in the seqA strain was not reversed in the mukBseqA double mutant, Membrane-as sociated nucleoids were isolated from the wild-type, mukB, seqA and mukBseq A strains and their sedimentation rates were compared under identical condi tions. Whereas the mukB mutation caused unfolding of the nucleoid, the seqA mutation led to a more compact packaging of the chromosome. The mukBseqA d ouble mutant regained the wild-type nucleoid organization as revealed from its rate of sedimentation. Microscopic appearances of the nucleoids were co nsistent with the sedimentation profiles, The mukB mutant was oversensitive to novobiocin and this susceptibility was suppressed in the mukBseqA strai n, suggesting possible roles of MukB and SeqA in maintaining chromosome top ology, The mutual phenotypic suppression of mukB and seqA alleles thus sugg ests that these genes have opposing influences on the organization of the b acterial nucleoid.