title: Intron conservation in a UV-specific DNA repair gene encoded by chlorella viruses.
abstract: Large dsDNA-containing chlorella viruses encode a pyrimidine dimer-specific glycosylase (PDG) that initiates repair of UV-induced pyrimidine dimers. The PDG enzyme is a homologue of the bacteriophage T4-encoded endonuclease V. The pdg gene was cloned and sequenced from 42 chlorella viruses isolated over 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 100% conserved among the viruses and another 4 viruses contain an 81-nucleotide intron, in the same position, that is nearly 100% identical (one virus differed by one base). In contrast, the nucleotides in the pdg coding regions (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 sequences which are characteristic of nuclear-located, spliceosomal processed pre-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 sequence in another 4 viruses imply strong selective pressure to maintain the DNA sequence of the intron when it is in the pdg gene. However, the ability of intron-plus and intron-minus viruses to repair UV-damaged DNA in the dark was nearly identical. These findings contradict the widely accepted dogma that intron sequences are more variable than exon sequences.
authors.affiliation: Department of Plant Pathology, University of Nebraska-Lincoln 68583-0722, USA.
authors.forename: L
authors.initials: L
authors.lastname: Sun
authors.forename: Y
authors.initials: Y
authors.lastname: Li
authors.forename: A K
authors.initials: AK
authors.lastname: McCullough
authors.forename: T G
authors.initials: TG
authors.lastname: Wood
authors.forename: R S
authors.initials: RS
authors.lastname: Lloyd
authors.forename: B
authors.initials: B
authors.lastname: Adams
authors.forename: J R
authors.initials: JR
authors.lastname: Gurnon
authors.forename: J L
authors.initials: JL
authors.lastname: Van Etten
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D017124:Conserved Sequence; D045647:DNA Glycosylases; D004260:DNA Repair / Q000235:genetics* / Q000528:radiation effects*; D005091:Exons; D014644:Genetic Variation; D007438:Introns; D008969:Molecular Sequence Data; D009699:N-Glycosyl Hydrolases / Q000235:genetics* / Q000378:metabolism; D017862:Phycodnaviridae / Q000235:genetics* / Q000528:radiation effects; D010802:Phylogeny; D014466:Ultraviolet Rays
chemical_list: D045647:DNA Glycosylases; D009699:N-Glycosyl Hydrolases; C036401:deoxyribopyrimidine endonucleosidase