DNA repair in Mycoplasma gallisepticum

Abstract

Background: DNA repair is essential for the maintenance of genome stability in all living beings. Genome size as well as the repertoire and abundance of DNA repair components may vary among prokaryotic species. The bacteria of the Mollicutes class feature a small genome size, absence of a cell wall, and a parasitic lifestyle. A small number of genes make Mollicutes a good model for a "minimal cell" concept.

Results: In this work we studied the DNA repair system of Mycoplasma gallisepticum on genomic, transcriptional, and proteomic levels. We detected 18 out of 22 members of the DNA repair system on a protein level. We found that abundance of the respective mRNAs is less than one per cell. We studied transcriptional response of DNA repair genes of M. gallisepticum at stress conditions including heat, osmotic, peroxide stresses, tetracycline and ciprofloxacin treatment, stationary phase and heat stress in stationary phase.

Conclusions: Based on comparative genomic study, we determined that the DNA repair system M. gallisepticum includes a sufficient set of proteins to provide a cell with functional nucleotide and base excision repair and mismatch repair. We identified SOS-response in M. gallisepticum on ciprofloxacin, which is a known SOS-inducer, tetracycline and heat stress in the absence of established regulators. Heat stress was found to be the strongest SOS-inducer. We found that upon transition to stationary phase of culture growth transcription of DNA repair genes decreases dramatically. Heat stress does not induce SOS-response in a stationary phase.

Figure 1

Domain organization of Exo protein of M. gallisepticum , M. genitalium, and M. pneumoniae in comparison to DNA polymerase I of E. coli and B. subtilis .

Figure 2

Transcription of genes of M. gallisepticum involved in DNA repair and homeostasis. A – Transcription profiles of DNA repair and homeostasis genes under different conditions. 23S rRNA gene was used as a reference. Genes with q-values less than 0.05 were considered to change the expression significantly. Colors indicate direction (red for upregulation, blue for downregulation) and level of expression change (log2). Gray indicates no statistically significant change in expression (t-test, BH-correction, q-value > 0.05); tet – tetracycline treatment, cfx – ciprofloxacin treatment. B – Kinetics of the transcriptional response during heat stress. Each box shows a different expression pattern (see Methods). Individual genes are shown by lines. Only genes that significantly change expression with at least one stress duration are shown. Gene expressions were normalized to mean zero and variance one before plotting. Distributions of normalized expressions for given stress duration and pattern are shown by boxes. Genes with the same rank in all conditions were considered to have similar expression patterns. Only genes with significant expression changes between control and at least one stress duration were used.

Figure 3

Hypothetical model of the MMR and BER systems in M. gallisepticum.