SOS-independent induction of dinB transcription by beta-lactam-mediated inhibition of cell wall synthesis in Escherichia coli

Abstract

Transcription of the dinB gene, encoding DNA polymerase IV, is induced by the inhibition of cell wall synthesis at different levels. Using the beta-lactam antibiotic ceftazidime, a PBP3 inhibitor, as a model, we have shown that this induction is independent of the LexA/RecA regulatory system. Induction of dinB transcription mediated by ceftazidime produces an increase in the reversion of a +1 Lac frameshift mutation.

FIG. 1.

Disk plate assays showing the effects of different antibiotics on the transcription of the dinB::lacZ operon fusion. (A) Antibiotics belonging to different families (the charge of each disk [in micrograms] is in parentheses) were as follows: on disk 1, ceftazidime (30); on disk 2, chloramphenicol (50); on disk 3, ciprofloxacin (5); on disk 4, erythromycin (30); on disk 5, tetracycline (50); on disk 6, rifampin (100); and on disk 7, amikacin (30). (B) Antibiotics affecting the cell wall synthesis at different steps (the charge of each disk [in micrograms] is in parentheses) were as follows: on disk 1, amoxicillin-clavulanate (30); on disk 2, imipenem (10); on disk 3, fosfomycin (50); on disk 4, no antibiotic; on disk 5, aztreonam (30); and on disk 6, d-cycloserine (100). All disks were purchased from Oxoid Ltd., except those containing rifampin and d-cycloserine, which were prepared in our laboratory.

FIG. 2.

Effects of the Δ(srlR-recA)306::Tn10 and lexA1 genetic backgrounds on the ceftazidime-mediated induction of transcription of the dinB::lacZ fusion. The curves on the left of each panel indicate the transcription levels (n-fold) between treated and nontreated cultures (calculated by assigning a value of 1 to the transcription level of the nontreated culture at each time point). Concentrations of CAZ (A) and NAL (B) expressed in micrograms per milliliter were as follows: 1 (♦), 2 (▴), 4 (▪), and 8 (•). The mean values of results from three independent experiments are displayed. The curves on the right of each panel represent the growth curves without and with CAZ or NAL for strain GW1030 (symbols are as described for the transcription levels; ○ indicates the values obtained without CAZ). Curves for recA and lexA1 derivatives are not presented because they are similar to those for strain GW1030. OD-600, optical density at 600 nm; wt, wild type.

FIG. 3.

Number of Lac⁺ revertants after CAZ treatment. Bars represent the mean numbers of Lac⁺ colonies per 10⁸ plated cells of CAZ-treated and nontreated SMR4562 (open bars) and its dinB10 derivative (filled bars) after 3 days of incubation. T bars represent standard deviations.