Radiation tolerance

The Medalia team analyzed chromatin organization and radio resistance in the Planctomycetes Gemmata obscuriglobus. They report that Gemmata tolerates high doses of UV and ionizing radiation. Using cryoelectron tomography they found a highly ordered condensed-chromatin organization and a complex network of double membranes engulfing the condensed DNA. The complex double-membrane system emanates from the internal cell membrane. There is some ambiguity in the paper since on one side, they report that their results imply that the bacterial nucleoid is not completely sealed by the double-membrane system but on the other side, they conclude that multiple nucleoid domains are enclosed by the double-membrane system. So is it enclosed or not completely sealed?

The analysis of radio resistance is interesting. They report that G. obscuriglobus is highly resistant to UVC radiations. They suggest that this is linked to the condensed stated of the nucleoid and conclude that their observations support the notion that packed chromatin organization enhances radiation tolerance. Their tomography is based on a 15-nm-thick slice, when a typical bacteria is around 3 to 5 microns. When E. coli dies around 300 J/m2 of UV dose, Gemmata can support around 3 times that, close to 900 J/m2. This high level of radio resistance could be linked to non-homologuous DNA end joining (NHEJ), a phenomenon linked to double strand break repair, meiosis recombination and to the VDJ locus rearrangement processes in eukaryotes. Bioinformatics investigation of the Gemmata proteome revealed genes with homology to those for RecA, RecB, and RecD (no RecC), as well as for NHEJ mechanism ATP-dependent DNA ligases. Wittingly, they highlight that the G. obscuriglobus DNA repair ligase protein is significantly smaller than its bacterial homologues (58.4 vs >80kDa in other bacteria), suggesting that this lighter mass enhances the accessibility to the DNA breaks within the condensed DNA environment. Of course, the connection between higher radiation tolerance and DNA condensation is still to be demonstrated but the arguments and data presented here makes it very likely in this organism. Thus, G. obscuriglobus has evolved versatile mechanisms to deal with stress conditions.