title: The phage T4 nrdB intron: a deletion mutant of a version found in the wild.
abstract: Bacteriophage T4 possesses three self-splicing group I introns. Two of the three introns are mobile elements; the third, in the gene encoding a subunit of the phage nucleotide reductase (nrdB), is not mobile. Because intron mobility offers a reasonable explanation for the paradoxical occurrence of large intervening sequences in a space-efficient eubacterial phage, it is puzzling that the nrdB intron is not mobile like its compatriots. We have discovered a larger nrdB intron in a closely related phage, and we infer from comparative sequence data that the T4 intron is a deletion mutant derived from this larger intron. This larger nrdB intron encodes an open reading frame of 269 codons, which we have cloned and overexpressed. The overexpressed protein shows a dsDNA endonuclease activity specific for the intronless nrdB gene, typical of mobile introns. Thus, we believe that all three introns of T4 are or were mobile "infectious introns" and that they have entered into and been maintained in the phage population by virtue of this efficient mobility.
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder 80309-0347.
authors.forename: S R
authors.initials: SR
authors.lastname: Eddy
authors.forename: L
authors.initials: L
authors.lastname: Gold
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002872:Chromosome Deletion; D003001:Cloning, Molecular; D004271:DNA, Fungal; D004706:Endodeoxyribonucleases / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D007438:Introns; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D016133:Polymerase Chain Reaction; D012331:RNA, Fungal; D012264:Ribonucleotide Reductases / Q000235:genetics*; D016415:Sequence Alignment; D013604:T-Phages / Q000201:enzymology / Q000235:genetics*
chemical_list: D004271:DNA, Fungal; D012331:RNA, Fungal; D012264:Ribonucleotide Reductases; D004706:Endodeoxyribonucleases