Abstract
The sequence of Vibrio cholerae genome revealed three genes belonging to the photolyase/cryptochrome blue-light photoreceptor family. The proteins encoded by the three genes were purified and characterized. All three proteins contain folate and flavin cofactors and have absorption peaks in the range of 350-500 nm. Only one of the three, VcPhr, is a photolyase specific for cyclobutane pyrimidine dimers. The other two are cryptochromes and were designated VcCry1 and VcCry2, respectively. Mutation of phr abolishes photoreactivation of UV-induced killing, whereas mutations in cry1 and cry2 do not affect photorepair activity. VcCry1 exhibits some unique features. Of all cryptochromes characterized to date, it is the only one that contains stoichiometric amounts of both chromophores and retains its flavin cofactor in the two-electron reduced FADH2 form. In addition, VcCry1 exhibits RNA binding activity and co-purifies with an RNA of 60-70 nucleotides in length.
Highlights
The photolyase/cryptochrome family encompasses blue-light photoreceptors, which use near UV/blue-light photons as an energy source to either repair far UV (200 –300 nm)-induced DNA lesions or to regulate growth and development in plants and synchronize the circadian clock with the daily light-dark cycles in animals (1–3)
In addition to their light-dependent functions, both photolyase and cryptochrome carry out some light-independent functions important for cellular physiology (4, 5); photolyase stimulates excision of UV damage and certain chemical lesions by nucleotide excision repair (4), and cryptochromes inhibit transcription of several circadian clock genes (6 – 8)
The authors suggested that the presence of three photolyase homologues, more than have been found in other bacterial species sequenced to date, may allow V. cholerae to photorepair the two major UV photoproducts, the cyclobutane pyrimidine dimer and the (6-4) photoproduct, by readily available light energy (13)
Summary
The photolyase/cryptochrome family encompasses blue-light photoreceptors, which use near UV/blue-light photons as an energy source to either repair far UV (200 –300 nm)-induced DNA lesions or to regulate growth and development in plants and synchronize the circadian clock with the daily light-dark cycles in animals (1–3) In addition to their light-dependent functions, both photolyase and cryptochrome carry out some light-independent functions important for cellular physiology (4, 5); photolyase stimulates excision of UV damage and certain chemical lesions by nucleotide excision repair (4), and cryptochromes inhibit transcription of several circadian clock genes (6 – 8). Genome sequencing projects have identified over 50 photolyase/cryptochrome candidate genes in various organisms including some animal viruses (2) In some instances, these genes have been shown to encode photolyases or cryptochromes using biochemical and genetic approaches. Because of a lack of biochemical data on a presumptive “cryptochrome photocycle,” the roles of cryptochromes as photoreceptors, in particular in animals, has been questioned (8)
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