DNA photolyase homologs are the major UV resistance factors in the cyanobacterium Synechocystis sp. PCC 6803
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O R I GI N A L P A P E R
W.-O. Ng á H. B. Pakrasi
DNA photolyase homologs are the major UV resistance factors in the cyanobacterium Synechocystis sp. PCC 6803
Received: 12 December 1999 / Accepted: 26 September 2000 / Published online: 12 December 2000 Ó Springer-Verlag 2000
Abstract In this study, the unicellular photosynthetic cyanobacterium Synechocystis sp. PCC 6803 was used as a model phototroph to study the contribution of enzymatic photoreactivation to the overall protection against UV irradiation. We have isolated genes encoding two DNA photolyase homologs, phrA and phrB, from Synechocystis 6803. phrA encodes an 8-hydroxy-5deazaribo¯avin (HDF) type, Class I DNA photolyase. By complementing a photolyase-de®cient mutant strain of Escherichia coli, we demonstrated that PhrA is a DNA photolyase. Analysis of a phrA knockout mutant strain suggested that this gene is responsible for the majority of the observed UV resistance in Synechocystis 6803. Similar studies on phrB demonstrated that it also contributes to photoreactivation, but to a much lesser degree. Based on these ®ndings, we conclude that enzymatic photoreactivation is the primary process used for repairing UV-induced damage in Synechocystis 6803. Key words Cyanobacteria á DNA photolyase á DNA repair á Photoreactivation á UV stress
Introduction Photoreactivation, an enzyme-mediated and lightdependent process that repairs DNA photoproducts induced by UV radiation (Sancar 1994), is the subject of the present study. It is known that this process greatly enhances the survival of cyanobacteria and many other organisms subjected to severe UV stress. The only known class of proteins involved in this process are the DNA photolyases. DNA photolyases and related proteins have
W.-O. Ng á H. B. Pakrasi (&) Department of Biology, Washington University, Campus Box 1137, One Brookings Drive, St. Louis, MO 63130, USA E-mail: [email protected] Tel.: +1-314-9356853 Fax: +1-314-9356803
been identi®ed in a wide range of organisms from eubacteria and Archaea to eukaryotes, including human and plants (Kanai et al. 1997). In human, DNA photolyase-like proteins do not encode DNA photolyase, but probably play a role in the entrainment of the circadian clock. In plants, genes encoding DNA photolyases speci®c for cyclobutane pyrimidine dimers (CPDs) (Ahmad et al. 1997) and pyrimidine (6-4) pyrimidone photoproducts [(6-4) photoproducts] (Britt et al. 1993; Nakajima et al. 1998) have recently been identi®ed. In addition, blue light receptors from plants and animals have been found to share a high degree of homology with DNA photolyases (Cashmore et al. 1999). The unicellular cyanobacterium Synechocystis sp. PCC 6803 oers an ideal experimental system for the study of enzymatic photoreactivation in a photosynthetic organism. This bacterium is naturally transformable, and is amenable to various genetic manipulations (Williams 1988). Moreover, Synechocystis 6803 is the ®rst phototrophic organism whose complete genome sequence has been determined (Kaneko et al. 1996). Cyan
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