Regulation of cell division in Escherichia coli: SOS induction and cellular location of the sulA protein, a key to lon-associated filamentation and death

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Abstract
Mutations in sulA (sfiA) block the filamentation and death of capR (lon) mutants that occur after treatments that damage DNA or inhibit DNA replication and thereby induce the SOS response. Previous sulA-lacZ gene fusion studies showed that sulA is transcriptionally regulated by the SOS response system (lexA/recA). SulA protein may be additionally regulated proteolytically through the capR (lon) protease, i.e., in lon mutants lacking a functional ATP-dependent protease there would be more SulA protein. A hypothesized function for SulA protein is an inhibitor of cell septation. To investigate aspects of this model, lon, lon sulA and lon sulB strains containing multicopy plasmids specifying the sulA+ gene were constructed. Multicopy sulA+ plasmids could not be established in lon strains because more SulA protein accumulates than in a lon+ strain. When the sulA gene was mutated by a mini-Mu transposon, the plasmid could be established in the lon strains. SulA+ plasmids could be established in lon+, lon sulA and lon sulB strains. The sulA+ plasmids caused lon sulA and lon sulB cells to exist as filaments without SOS induction and to be sensitive to UV light and nitrofurantoin. Evidence implicated higher basal levels of SulA protein in these lon plasmid sulA+ strains as the cause of filamentation. It was confirmed that the SulA protein is an 18-kilodalton polypeptide that can be induced by treatment with nalidixic acid. The SulA protein was rapidly degraded in a lon+ strain, but was comparatively more stable in vivo in a lon sulB mutant. The SulA protein was localized to the membrane by several techniques.