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The role of the genomic bipyrimidine nucleotide frequency in pyrimidine dimer formation caused by germicidal UV radiation was studied in three microbial reference organisms (Escherichia coli K12, Deinococcus radiodurans R1, spores and cells of Bacillus subtilis 168). The sensitive HPLC tandem mass spectrometry assay was used to identify and quantify the different bipyrimidine photoproducts induced in the DNA of microorganisms by germicidal UV radiation. The yields of photoproducts per applied fluence were very similar among vegetative cells but twofold reduced in spores. This similarity in DNA photoreactivity greatly contrasted with the 11-fold range determined in the fluence causing a decimal reduction of survival. It was also found that the spectrum of UV-induced bipyrimidine lesions was species-specific and the formation rates of bi-thymine and bi-cytosine photoproducts correlated with the genomic frequencies of thymine and cytosine dinucleotides in the bacterial model systems.