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The N(6)-(isopentenyl)adenosine (i(6)A) modification of some tRNAs at position A37 is found in all kingdoms and facilitates codon-specific mRNA decoding, but occurs in different subsets of tRNAs in different species. Here we examine yeasts' tRNA isopentenyltransferases (i.e., dimethylallyltransferase, DMATase, members of the Δ(2)-isopentenylpyrophosphate transferase, IPPT superfamily) encoded by tit1(+) in Schizosaccharomyces pombe and MOD5 in Saccharomyces cerevisiae, whose homologs are Escherichia coli miaA, the human tumor suppressor TRIT1, and the Caenorhabditis elegans life-span gene product GRO-1. A major determinant of miaA activity is known to be the single-stranded tRNA sequence, A36A37A38, in a stem-loop. tRNA(Trp)(CCA) from either yeast is a Tit1p substrate, but neither is a Mod5p substrate despite the presence of A36A37A38. We show that Tit1p accommodates a broader range of substrates than Mod5p. tRNA(Trp)(CCA) is distinct from Mod5p substrates, which we sort into two classes based on the presence of G at position 34 and other elements. A single substitution of C34 to G converts tRNA(Trp)(CCA) to a Mod5p substrate in vitro and in vivo, consistent with amino acid contacts to G34 in existing Mod5p-tRNA(Cys)(GCA) crystal structures. Mutation of Mod5p in its G34 recognition loop region debilitates it differentially for its G34 (class I) substrates. Multiple alignments reveal that the G34 recognition loop sequence of Mod5p differs significantly from Tit1p, which more resembles human TRIT1 and other DMATases. We show that TRIT1 can also modify tRNA(Trp)(CCA) consistent with broad recognition similar to Tit1p. This study illustrates previously unappreciated molecular plasticity and biological diversity of the tRNA-isopentenyltransferase system of eukaryotes. 相似文献
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Detecting recombination from gene trees 总被引:28,自引:10,他引:18
In this article, a method is proposed for detecting recombination in the
sequences of a gene from a set of closely related organisms. The method,
the Homoplasy Test, is appropriate when the sequences are rather similar,
differing by 1%-5% of nucleotides. It is effective in detecting relatively
frequent recombination between a set of rather similar strains, in contrast
to previous methods which detect rare or unique transfers between more
distant strains. It is based on the fact that, if there is no recombination
and if no repeated mutations have occurred (homoplasy), then the number of
polymorphic sites, v, is equal to the number of steps, t, in a
most-parsimonious tree. If the number of "apparent homoplasies" in the
most-parsimonious tree, h = t-v, is greater than zero, then either
homoplasies have occurred by mutation or there has been recombination. An
estimate of the distribution of h expected on the null hypothesis of no
recombination depends on Se, the "effective site number," defined as
follows: if ps is the probability that two independent substitutions in the
gene occur at the same site, then Se = 1/ps. Se can be estimated if a
suitable outgroup is available. The Homoplasy Test is applied to three
bacterial genes and to simulated gene trees with varying amounts of
recombination. Methods of estimating the rate, as opposed to the
occurrence, of recombination are discussed.
相似文献
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Alyssa M. Weinrauch Tamzin A. Blewett Chris N. Glover Greg G. Goss 《Journal of fish biology》2019,95(6):1471-1479
This study used 3H-L-alanyl-L-alanine to demonstrate dipeptide uptake using in vitro gut sacs prepared from the hindgut of the Pacific hagfish Eptatretus stoutii. Concentration-dependent kinetic analysis resulted in a sigmoidal distribution with a maximal (± SE) uptake rate (Jmax-like) of 70 ± 3 nmol cm−2 h−1 and an affinity constant (Km-like) of 1072 ± 81 μM. Addition of high alanine concentrations to transport assays did not change dipeptide transport rates, indicating that hydrolysis of the dipeptide in mucosal solutions and subsequent uptake via apical amino acid transporters was not occurring, which was further supported by a Km distinct from that of amino acid transport. Transport occurred independent of mucosal pH, but uptake was reduced by 42% in low mucosal sodium. This may implicate cooperation between peptide transporters and sodium-proton exchangers, previously demonstrated in several mammalian and teleost species. Finally, apical L-alanyl-L-alanine uptake rates (i.e., mucosal disappearance) were significantly increased following a meal, demonstrating regulation of uptake. Overall, this examination of dipeptide acquisition in the earliest extant Agnathan suggests evolutionarily conserved mechanisms of transport between hagfish and later-diverging vertebrates such as teleosts and mammals. 相似文献