title: Factors limiting SOS expression in log-phase cells of Escherichia coli.
abstract: In Escherichia coli, RecA-single-stranded DNA (RecA-ssDNA) filaments catalyze DNA repair, recombination, and induction of the SOS response. It has been shown that, while many (15 to 25%) log-phase cells have RecA filaments, few (about 1%) are induced for SOS. It is hypothesized that RecA's ability to induce SOS expression in log-phase cells is repressed because of the potentially detrimental effects of SOS mutagenesis. To test this, mutations were sought to produce a population where the number of cells with SOS expression more closely equaled the number of RecA filaments. Here, it is shown that deleting radA (important for resolution of recombination structures) and increasing recA transcription 2- to 3-fold with a recAo1403 operator mutation act independently to minimally satisfy this condition. This allows 24% of mutant cells to have elevated levels of SOS expression, a percentage similar to that of cells with RecA-green fluorescent protein (RecA-GFP) foci. In an xthA (exonuclease III gene) mutant where there are 3-fold more RecA loading events, recX (a destabilizer of RecA filaments) must be additionally deleted to achieve a population of cells where the percentage having elevated SOS expression (91%) nearly equals the percentage with at least one RecA-GFP focus (83%). It is proposed that, in the xthA mutant, there are three independent mechanisms that repress SOS expression in log-phase cells. These are the rapid processing of RecA filaments by RadA, maintaining the concentration of RecA below a critical level, and the destabilizing of RecA filaments by RecX. Only the first two mechanisms operate independently in a wild-type cell.
authors: Department of Microbiology, Morrill Science Center IV N203, University of Massachusetts at Amherst, Amherst, Massachusetts, USA.
authors: Shawn C
authors: SC
authors: Massoni
authors: Michael C
authors: MC
authors: Leeson
authors: Jarukit Edward
authors: JE
authors: Long
authors: Kristin
authors: K
authors: Gemme
authors: Alice
authors: A
authors: Mui
authors: Steven J
authors: SJ
authors: Sandler
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D002455:Cell Division; D053904:DNA Breaks, Single-Stranded; D004269:DNA, Bacterial / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000166:cytology / Q000235:genetics / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial / Q000502:physiology; D011485:Protein Binding; D013014:SOS Response, Genetics / Q000502:physiology*
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; C100901:RadA protein, bacteria