We report a remarkably high UV-radiation resistance in the extremely halophilic archaeon Halobacterium NRC-1 withstanding up to 110 J/m 2 with no loss of viability. Gene knockout analysis in two putative photolyase-like genes (phr1 and phr2) implicated only phr2 in photoreactivation. The UV-response was further characterized by analyzing simultaneously, along with gene function and protein interactions inferred through comparative genomics approaches, mRNA changes for all 2400 genes during light and dark repair. In addition to photoreactivation, three other putative repair mechanisms were identified including d(CTAG) methylation-directed mismatch repair, four oxidative damage repair enzymes, and two proteases for eliminating damaged proteins. Moreover, a UV-induced down-regulation of many important metabolic functions was observed during light repair and seems to be a phenomenon shared by all three domains of life. The systems analysis has facilitated the assignment of putative functions to 26 of 33 key proteins in the UV response through sequence-based methods and/or similarities of their predicted three-dimensional structures to known structures in the PDB. Finally, the systems analysis has raised, through the integration of experimentally determined and computationally inferred data, many experimentally testable hypotheses that describe the metabolic and regulatory networks of Halobacterium NRC-1.[Supplemental material is available online at www.genome.org. The sequence data from this study have been submitted to GEO under accession no. GSE1040.]Biological systems have evolved mechanisms to appropriately respond to environmental stresses that can damage proteins and DNA. Halobacterium NRC-1, for example, has evolved the capacity to withstand up to 4.5M salinity, ultraviolet radiation (UV), and oxidative stress in its natural environment. One such adaptation, the high-density of acidic residues on the surface of almost all halobacterial proteins, is believed to stabilize protein structure and function in high salt (Kennedy et al. 2001). It can also physically relocate to favorable environments using sensors for light, oxygen, and nutrients, and produce energy through respiration, fermentation, or phototrophy (Ng et al. 2000). We have previously reported a response mechanism in Halobacterium NRC-1 that coordinately regulates phototrophy and arginine fermentation, two major sources for anaerobic energy production. The effects of perturbing the function of a regulator in this response mechanism were observed throughout the metabolic network (Baliga et al. 2002).Systems approaches allow us to raise a whole series of hypotheses regarding the behavior of a system in response to a defined experimental perturbation. These hypotheses then stimulate iterative cycles of perturbations and analyses to verify these hypotheses. Here, we report a systems-level study on the behavior of Halobacterium NRC-1 upon UV-C irradiation. Shortwave ultraviolet light (UV-C) induces two types of mutagenic lesions in DNA-cyclobutane pyrimid...

We report that the halophilic archaeon Halobacterium sp. strain NRC-1 is highly resistant to desiccation, high vacuum and 60Co gamma irradiation. Halobacterium sp. was able to repair extensive double strand DNA breaks (DSBs) in its genomic DNA, produced both by desiccation and by gamma irradiation, within hours of damage induction. We propose that resistance to high vacuum and 60Co gamma irradiation is a consequence of its adaptation to desiccating conditions. Gamma resistance in Halobacterium sp. was dependent on growth stage with cultures in earlier stages exhibiting higher resistance. Membrane pigments, specifically bacterioruberin, offered protection against cellular damages induced by high doses (5 kGy) of gamma irradiation. High-salt conditions were found to create a protective environment against gamma irradiation in vivo by comparing the amount of DSBs induced by ionizing radiation in the chromosomal DNA of Halobacterium sp. to that of the more radiation-sensitive Escherichia coli that grows in lower-salt conditions. No inducible response was observed after exposing Halobacterium sp. to a nonlethal dose (0.5 kGy) of gamma ray and subsequently exposing the cells to either a high dose (5 kGy) of gamma ray or desiccating conditions. We find that the hypersaline environment in which Halobacterium sp. flourishes is a fundamental factor for its resistance to desiccation, damaging radiation and high vacuum.

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Cocoyam (Colocasia esculenta) corms, purchased from a local barn in Awka, Anambra state, showing spoilage symptoms were examined for rot associated with fungal pathogens. The spoilage organisms were isolated from the corms by explanting the spoilt tissues obtained aseptically onto Sabouraud Dextrose Agar supplemented with chloramphenicol an antibacterial agent. The developing isolates were purified by repeated subculture and identified macroscopically and microscopically using the slide culture technique. The organisms recovered from spoilt cocoyam were Aspergillus niger, Rhizopus stolonifer, Mucor circinelloides, Penicillium cyclopium and Fusarium oxysporum. The rot due to Rhizopus, Mucor and Fusarium were extensive resulting in complete maceration of cocoyam tissues. The pathogenicity test also showed that the above organisms were responsible for cocoyam spoilage. These spoilage organisms may have had access into these cocoyam corms through air, wounds, working equipment (for harvesting) and pests. Proper care should be taken in the handling of these corms as some of these spoilage fungi are known to have negative impact on both humans and livestock because they produce mycotoxins.