The Essential Role of the Deinococcus radiodurans ssb Gene in Cell Survival and Radiation Tolerance

Abstract

Recent evidence has implicated single-stranded DNA-binding protein (SSB) expression level as an important factor in microbial radiation resistance. The genome of the extremely radiation resistant bacterium Deinococcus radiodurans contains genes for two SSB homologs: the homodimeric, canonical Ssb, encoded by the gene ssb, and a novel pentameric protein encoded by the gene ddrB. ddrB is highly induced upon exposure to radiation, and deletions result in decreased radiation-resistance, suggesting an integral role of the protein in the extreme resistance exhibited by this organism. Although expression of ssb is also induced after irradiation, Ssb is thought to be involved primarily in replication. In this study, we demonstrate that Ssb in D. radiodurans is essential for cell survival. The lethality of an ssb deletion cannot be complemented by providing ddrB in trans. In addition, the radiation-sensitive phenotype conferred by a ddrB deletion is not alleviated by providing ssb in trans. By altering expression of the ssb gene, we also show that lower levels of transcription are required for optimal growth than are necessary for high radiation resistance. When expression is reduced to that of E. coli, ionizing radiation resistance is similarly reduced. UV resistance is also decreased under low ssb transcript levels where growth is unimpaired. These results indicate that the expression of ssb is a key component of both normal cellular metabolism as well as pathways responsible for the high radiation tolerance of D. radiodurans.

Real-time submillisecond single-molecule FRET dynamics of freely diffusing molecules with liposome tethering

“…The fast migration and low binding energy of dr SSB should accelerate dr SSB's redistribution on ssDNA and removal from ssDNA for downstream DNA repair processes, compared with those of E. coli SSB. This may provide one of the clues for the explanation why D. radiodurans has a high tolerance to the DNA-damaging environment 59 .…”

Single Strand Annealing Plays a Major Role in RecA-Independent Recombination between Repeated Sequences in the Radioresistant Deinococcus radiodurans Bacterium

“…In contrast, the single strand annealing activity of DdrB is not stimulated but rather inhibited by inclusion of the SSB protein in the in vitro annealing reaction [ 4 , 6 ]. The deinococcal SSB protein is an essential protein and the DdrB protein is unable, even when overexpressed, to replace SSB for cell viability [ 41 ]. SSB is crucial for all aspects of DNA metabolism [ 42 ] while DdrB seems to have a more specialized role in DNA repair and plasmid transformation by stimulating the SSA pathway.…”

Polyploidy in Archaea and Bacteria: About Desiccation Resistance, Giant Cell Size, Long-Term Survival, Enforcement by a Eukaryotic Host and Additional Aspects

“…Two single-strand DNA-binding proteins turned out to be of specific importance, i.e. the general SSB, which has an unusual high constitutive level in D. radiodurans , and the Deinococcus -specific DdrB, which is specifically produced after the introduction of double-strand breaks [Lockhart and DeVeaux, 2013] and has an early function in double-strand break repair [Bouthier de la Tour et al, 2011]. The importance of single-strand binding proteins for the resistance against double-strand breaks seems to be universal because it has also been reported for the ar-chaeon Halobacterium salinarum [DeVeaux et al, 2007] and the eukaryote Schizosaccharomyces pombe [Cavero et al, 2010].…”