A thiol/disulfide oxidoreductase component of the GSH system, glutaredoxin (Grx), is involved in the reduction of GSH-based mixed disulfides and participates in a variety of cellular redox pathways. A single cytosolic Grx (Grx1) was previously described in mammals. We now report identification and characterization of a second mammalian Grx, designated Grx2. Grx2 exhibited 36% identity with Grx1 and had a disulfide active center containing the Cys-Ser-Tyr-Cys motif. Grx2 was encoded in the genomes of mammals and birds and expressed in a variety of cell types. The gene for human Grx2 consisted of four exons and three introns, spanned 10 kilobase pairs, and localized to chromosome 1q31.2-31.3. The coding sequence was present in all exons, with the first exon encoding a mitochondrial signal peptide. The mitochondrial leader sequence was also present in mouse and rat Grx2 sequences and was shown to direct either Grx2 or green fluorescent protein to mitochondria. Alternative splicing forms of mammalian Grx2 mRNAs were identified that differed in sequences upstream of exon 2. To functionally characterize the new protein, human and mouse Grx2 proteins were expressed in Escherichia coli, and the purified proteins were shown to reduce mixed disulfides formed between GSH and S-sulfocysteine, hydroxyethyldisulfide, or cystine. Grx1 and Grx2 were sensitive to inactivation by iodoacetamide and H 2 O 2 and exhibited similar pH dependence of catalytic activity. However, H 2 O 2 -inactivated Grx2 could only be reactivated with 5 mM GSH, whereas Grx1 could also be reactivated with dithiothreitol or thioredoxin/thioredoxin reductase. The Grx2 structural model suggested a common reaction mechanism for this class of proteins. The data provide the first example of a mitochondrial Grx and also indicate the occurrence of a second functional Grx in mammals.

Mammalian selenium-containing proteins identified thus far contain selenium in the form of a selenocysteine residue encoded by UGA. These proteins lack common amino acid sequence motifs, but 3-untranslated regions of selenoprotein genes contain a common stem-loop structure, selenocysteine insertion sequence (SECIS) element, that is necessary for decoding UGA as selenocysteine rather than a stop signal. We describe here a computer program, SECISearch, that identifies mammalian selenoprotein genes by recognizing SECIS elements on the basis of their primary and secondary structures and free energy requirements. When SECISearch was applied to search human dbEST, two new mammalian selenoproteins, designated SelT and SelR, were identified. We determined their cDNA sequences and expressed them in a monkey cell line as fusion proteins with a green fluorescent protein. Incorporation of selenium into new proteins was confirmed by metabolic labeling with 75 Se, and expression of SelT was additionally documented in immunoblot assays. SelT and SelR did not have homology to previously characterized proteins, but their putative homologs were detected in various organisms. SelR homologs were present in every organism characterized by complete genome sequencing. The data suggest applicability of SECISearch for identification of new selenoprotein genes in nucleotide data bases.

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The pathogenesis of prion diseases is characterized by the accumulation of amyloid-like rods or scrapie-associated fibrils. The major protein component of scrapie-associated fibrils is an abnormally folded isoform of the normal cellular prion protein (PrP(C)) that is resistant to digestion by proteinase K and is referred to as PrP(Sc). Purified human recombinant (hr PrP) was used to characterize the binding of a set of RNAs with affinity to PrP proteins. We report here that hr PrP has two RNA-binding activities at physiological pH. One activity is capable of binding all of the screened RNAs with high affinity, whereas the other activity can bind only to a subset of the RNAs with high affinity in the presence of non-specific competitor RNAs. A novel RNA belonging to the latter class, RQ11+12, bound to hr PrP with high affinity in the presence of vast molar excesses of competing RNAs. Beads impregnated with the RQ11+12 RNA were used to construct a filtration column. The column efficiently bound hr PrP and native PrP(C) from serum and urine. Importantly, the filtration device was also capable of binding proteinase K-treated PrP(Sc) from serum and urine. The level of sensitivity of detection of PrP by standard Western blotting was increased at least 1000-fold by first concentrating PrP from solution with the filtration column.

The bacteriophage T4 segE gene encoding sitespecific endonuclease lies between the hoc.l and uvsW genes. The similar region of T-even-related phage RB30 lacks the segE gene. Here we demonstrate that the phage T4 segE gene is inherited preferably by progeny of mixed infection with RB30. The preferred inheritance of the segE gene depends on its own expression and is based on a non-reciprocal homologous recombination event providing the transfer of the gene from the segE-containing to the .se^E-lacking alíele. The SegE endonuclease cleaves DNA in a site located at the 5' end of the KVSW gene in the RB30 genome. The T4 DNA is also cleaved by the enzyme, but less efficiently. The cleavage at the RB30 site appears to initiate the observed conversion, which is stimulated by DNA homology and accompanied by co-conversion of flanking markers. Our findings provide a novel example of endonuclease-dependent generation of genetic variation in prokaryotes.

Homing endonucleases initiate nonreciprocal transfer of DNA segments containing their own genes and the flanking sequences by cleaving the recipient DNA. Bacteriophage T4 segB gene, which is located in a cluster of tRNA genes, encodes a protein of unknown function, homologous to homing endonucleases of the GIY-YIG family. We demonstrate that SegB protein is a site-specific endonuclease, which produces mostly 3′ 2-nt protruding ends at its DNA cleavage site. Analysis of SegB cleavage sites suggests that SegB recognizes a 27-bp sequence. It contains 11-bp conserved sequence, which corresponds to a conserved motif of tRNA TψC stem-loop, whereas the remainder of the recognition site is rather degenerate. T4-related phages T2L, RB1 and RB3 contain tRNA gene regions that are homologous to that of phage T4 but lack segB gene and several tRNA genes. In co-infections of phages T4 and T2L, segB gene is inherited with nearly 100% of efficiency. The preferred inheritance depends absolutely on the segB gene integrity and is accompanied by the loss of the T2L tRNA gene region markers. We suggest that SegB is a homing endonuclease that functions to ensure spreading of its own gene and the surrounding tRNA genes among T4-related phages.

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