Intron conservation in a UV-specific DNA repair gene encoded by chlorella viruses

Large dsDNA-containing chlorella viruses encode a pyrimidine dimer-specific glycosylase (PDG) that initiates repair of W-induced pyrimidine dimers, Th e PDG enzyme is a homologue of the bacteriophage T4-encoded endonuclease V. The pdg gene was cloned and sequenced from 42 chlorella viruses isolated o ver a 12-year period from diverse geographic regions. Surprisingly, the pdg gene from 15 of these 42 viruses contain a 98-nucleotide intron that is 10 0% conserved among the viruses and another 4 viruses contain an 81-nucleoti de intron, in the same position, that is nearly 100% identical (one virus d iffered by one base). In contrast, the nucleotides in the pdg coding region s (exons) from the intron-containing viruses are 84 to 100% identical. The introns in the pdg gene have 5'-AG/GTATGT and 3'-TTGCAG/AA splice site sequ ences which are characteristic of nuclear-located, spliceosomal processed p re-mRNA introns. The 100% identity of the 98-nucleotide intron sequence in the 15 viruses and the near-perfect identity of an 81-nucleotide intron seq uence in another it viruses imply strong selective pressure to maintain the DNA sequence of the intron when it is in the pdg gene. However, the abilit y of intron-plus and intron-minus viruses to repair UV-damaged DNA in the d ark was nearly identical. These findings contradict the widely accepted dog ma that intron sequences are more variable than exon sequences.