After incorporation of random nucleotides by TdT during heav

After incorporation of random sanguinarine mg by TdT during heavy-chain rearrangements, both TdT and pol λ may perform in trans polymerase activity (in unknown proportions), whereas synthesis of the complementary strand can only be achieved by pol λ using its gap-filling activity, which TdT lacks because of its Loop1 (Figure 4). In light-chain gene rearrangements Pol μ can perform not only template-independent, but also in trans polymerase and gap-filling activities (Figure 4).
Perspectives and conclusion Due to its ability to add random sequences to a DNA primer, TdT is an intrinsically ‘unpredictable’ polymerase (rather than a ‘misguided’ one [12]). This explains its tight regulation both in time and space, in order to restrict its use to V(D)J recombination. Here we suggest that not only its recruitment but also its gradual stop is programmed, by switching to a previously unsuspected templating mode across strand breaks after the addition of 4–5 random nucleotides. This would be due to the spatial constraints of the architecture of the whole NHEJ apparatus, a very active field in structural biology that achieved impressive progress recently, first for the structure of the ligation complex [], and more recently for the structure of the huge loading complex of NHEJ [93••, 94••, 95••]. It is known that TdT interacts with Ku heterodimer through its BRCT domain, as does pol μ [13, 14]. If the interaction of the BRCT domain with Ku heterodimer could be mapped, then it would be possible to place the polX with respect to the DNA–PKcs–DNA complex and thereby to shed light on spatial constraints at work. On the evolutionary level it would be interesting to do it for both pol mu and TdT, so as to assess how similar are the positioning of TdT and pol μ in this integrated view of the NHEJ complex. From Figure 1, it appears that TdT has lost just the 5′ phosphate binding site of pol mu and that its Loop1 is of the same length, but with a different sequence. Regarding Loop1 and its vexing property of escaping structural characterization in pol mu, we expect that the structure of the TdT chimera containing Loop1 of pol mu will inform us on its conformation and also allow, eventually, a comparison with the LigD polymerase that performs NHEJ in bacteria.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:
Acknowledgements
Introduction Graphene-based nanocomposites possess functional properties that cannot be realized in conventional composites or other carbon nanocomposites [1], [2], [3]. Typical investigations of these materials are focused on developing novel devices for sensing [4], [5], [6], [7] and EMI shielding [8], due to their enhanced thermal and electrical conductivities [9], [10]. In particular, nanocomposites with flexible polymer matrix offer interesting solutions for applications where the adaptability to a particular shape is one of the functional requirements [11], [12], [13]. Radiation sensing is another potential application of graphene-based nanocomposites [4], [14], [15], especially if they can be integrated in wearable devices, allowing for their use by astronauts during extra vehicular activities (EVA) or by operators exposed to radiation-contaminated environments on Earth. Ultraviolet radiation is well-known to cause damage to materials [16], [17], [18] and biological systems [19], [20], and poses serious risks in all environments where radiations are needed for technical applications (e.g. sterilization and surface modifications facilities) and, most significantly, in space environment [21], [22]. Exposure to UV radiation can generate important physical and chemical changes altering the original structure and properties of materials. In particular, the high energy UV-C band, with wavelengths shorter than 280 nm, is associated with one of the most damaging radiation exposure. One hour of UV-C irradiation at 254 nm can damage DNA strands [23], breaking chemical bonds in the nucleic acid sequence and creating new intramolecular bonds that change the DNA native structure [24], [25].