Influenza A viruses are usually non-pathogenic in wild aquatic birds, their natural reservoir. However, from May to July 2005, at Qinghai Lake in China, an unprecedented outbreak of highly pathogenic H5N1 avian influenza virus caused the death of thousands of wild migratory waterbirds. Herein, H5N1 influenza virus from bar-headed geese collected during the outbreak was characterized. Genomic analysis showed that A/Bar-headed Goose/Qinghai/0510/05 (Bh H5N1 virus) is a reassortant virus. Amino acid residue (lysine) at position 627 in the PB2 gene of the Bh H5N1 virus was the same as that of the human H5N1 virus (A/HK/483/97) and different from that of H5N1 avian influenza viruses deposited in GenBank. Antigenic analysis showed that significant antigenic variation has occurred in the Bh H5N1 virus. The Bh H5N1 virus induced systemic infections and caused 100 % mortality in chickens and mice, and 80 % mortality in ducks and geese. Bh H5N1 virus titres were higher in multiple organs of chickens, ducks and geese than in mice, and caused more severe histological lesions in chickens, ducks and mice than in geese. These results support the need to pay close attention and create control programmes to prevent the transmission of highly pathogenic avian influenza virus from wild migratory waterbirds into domestic chickens, ducks, geese and mammalian hosts.
Uracil DNA glycosylases (UDGs) play an important role in removing uracil from DNA to initiate DNA base excision repair. Here, we first characterized biochemically a thermostable UDG from the hyperthermophilic euryarchaeon Thermococcus barophilus Ch5 (Tba UDG), and probed its mechanism by mutational analysis.The recombinant Tba UDG cleaves specifically uracil-containing ssDNA and dsDNA at 65 o C. The enzyme displays an optimal cleavage activity at 55-75 o C. Tba UDG cleaves DNA over a wide pH spectrum ranging from 4.0 to 9.0 with an optimal pH of 5.0-8.0. In addition, the Tba UDG activity is independent on a divalent metal ion; however, both Zn 2+ and Cu 2+ completely inhibits the enzyme activity. Furthermore, the Tba UDG activity is also inhibited by high NaCl concentration. Tba UDG removes uracil from DNA by the order: U≈U/G>U/T≈U/C>U/A. The mutational studies showed that both the E118A and N159A mutants completely abolish the cleavage activity and retain the compromised binding activity, suggesting that residues E118 and N159 in Tba UDG are important for uracil recognition and removal.Our work provides a basis for determining the role of Tba UDG in the base excision repair pathway for uracil repair in Thermococcus.
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