Protection against solar UV radiations in organisms includes production of secondary metabolites such as pigments that can absorb UV and visible light. In this study, vioA gene coding for tryptophan 2-monooxygenase involved in the biosynthetic pathway of violacein-like purple violet pigment (PVP) was cloned and sequenced from an Antarctic bacterium Janthinobacterium sp. Ant5-2. A non-pigmented vioA mutant strain was obtained following mini-Tn10 transposon mutagenesis that produced a lesser amount of extrapolymeric substances (EPS) and exhibited a compromised survival to freeze-thaw cycles at 4˚C. Survival of vioA mutant strain during exponential and stationary growth phase was significantly reduced when exposed to UVB (320 nm) and UVC (254 nm) (dose range: 0-300 J/m²) light. Decreased viability of the mutant strain was noticed at 4˚C upon nutrient deprivation and exposure to UV light. The pigment production was augmented in surviving cells following increased doses of both UVB and UVC. Interestingly, at higher doses i.e. 100 and 300 J/m², pigmented wild-type strain showed better survival to harmful UVC than UVB. This could be attributed to the pigment structure, which has absorption maxima in the visible (575 nm) as well as in the UVC (270 nm) range. In conclusion, this study defines the photoprotective role of the PVP in Janthinobacterium sp. Ant5-2 against UV radiation using single gene mutation. Perhaps the photoprotective role of PVP in this bacterium is one of the key adaptive features that define their survival in Antarctic extreme conditions, especially during austral summer months.
The high energy photons in solar UV radiation are considered to be the most damaging component to the biological macromolecules (predominantly, protein, nucleic acids and lipid) in living cells. Most of the solar radiation energy transmitted to earth is composed of infrared (~37%), visible (~43%), and UV (~7%) wavelengths (Nicholson et al., 2005). Based on a “photobiological” definition of UV (the Global Solar UV Index), it is divided into three categories: UVC (200–280 nm), UVB (280–315 nm), and UVA (315–400 nm) (WHO, 2002). On Earth, the most of the solar UV radiation is attenuated by the atmospheric gases, pressure, suspended dust, water ices, etc. As a result, a relatively small amount of UV reaches the earth’s surface (Tomasko et al., 1999). Studies in Antarctica showed that harmful UVC radiations at 254 nm wavelength is attenuated by 40% and 90% at depths of 1 cm and 5 cm ice, respectively (Cockell & Horneck, 2001; Nienow et al., 1988). Ozone layer formed in the Earth’s stratosphere act as a shield by absorbing UV wavelengths shorter than ~300 nm (Warneck, 1988). The depletion of ozone layer has affected the spectral composition of solar UV radiation reaching Antarctica and Southern Ocean. In spring, increased levels of short wavelength UVB (280-320 nm) radiation was recorded in Antarctica (Staehelin et al., 2001).
It has been...