title: pH dependence of self-splicing by the group IA2 intron in a pre-mRNA derived from the nrdB gene of bacteriophage T4.
abstract: The nrdB gene of bacteriophage T4 contains a group IA2 intron. We have investigated the kinetics of self-splicing by a shortened variant of nrdB pre-mRNA in the presence of the co-substrates guanosine and 2'-amino-2'-deoxyguanosine. The pH dependence of the first transesterification step displayed parallel linear correlations for the two different co-substrates up to pH 7, above which the reaction with guanosine levels off to become pH independent. The plot for the 30-fold slower reaction with 2'-aminoguanosine is linear up to pH 8-8.5 and then levels off. The linear correlations with slopes close to unity suggest that a deprotonation event accelerates the transesterification reaction and that a change in rate limiting step occurs at a first order rate constant of approximately 1 min-1(i.e. for our system k cat/ K m approximately 10(5) M-1 min-1). The pH dependence of observed rate constants in different divalent metal ion mixtures, where the 2'-aminoguanosine-dependent reaction is enhanced 6- and 35-fold compared with that in magnesium, strongly supports this conclusion. This is, to our knowledge, the first report on an intact self-splicing group I intron where use of different co-substrates and divalent metal ions shows that a deprotonation enhances the rate and verifies that the transitions occurring during splicing of group I introns are all part of a common reaction sequence.
authors.affiliation: Department of Molecular Biology, Stockholm University, S-10691 Stockholm, Sweden.
authors.forename: A S
authors.initials: AS
authors.lastname: Sjögren
authors.forename: R
authors.initials: R
authors.lastname: Strömberg
authors.forename: B M
authors.initials: BM
authors.lastname: Sjöberg
mesh_terms: D017122:Bacteriophage T4 / Q000235:genetics*; D003849:Deoxyguanosine / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D004951:Esterification; D006151:Guanosine / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007438:Introns; D008274:Magnesium / Q000494:pharmacology; D009690:Nucleic Acid Conformation; D012322:RNA Precursors / Q000378:metabolism*; D012326:RNA Splicing; D012333:RNA, Messenger / Q000378:metabolism*; D012367:RNA, Viral / Q000378:metabolism; D012262:Ribonucleoside Diphosphate Reductase / Q000235:genetics*
chemical_list: D012322:RNA Precursors; D012333:RNA, Messenger; D012367:RNA, Viral; D006151:Guanosine; C014319:2'-amino-2'-deoxyguanosine; D012262:Ribonucleoside Diphosphate Reductase; D003849:Deoxyguanosine; D008274:Magnesium