Immunoglobulin A1 proteases: a structure–function update

IgA1 (immunoglobulin A1) antibodies are the first line of defence against microbial pathogens such as Neisseria meningitidis and Haemophilus influenzae. However, these bacteria secrete a site-specific protease that is capable of cleaving human IgA1 and interacting with other host components. The IgA proteases are released by the type V secretion pathway, which involves translocation through two membranes and an autolytic, post-translational processing step. Results reported recently throw light on the type V secretion pathway and on the roles of the multifunctional IgA protease. The IgA1 protease-recognition sequence is present within the IgA1 hinge region as well as in the variable sequence connecting the IgA1 protease to its translocator domain. Recent results suggest that neisserial IgA1 proteases are capable of cleaving substrates lacking the classical recognition sequence. This review will cover recent advances in the IgA protease field.

Acres of antibodies: the future of recombinant biomolecule production?

The Cambridge Healthtech Institute's second international conference entitled "Recombinant Therapeutics: Antibodies and Biomolecules" was held in Baltimore, MD, USA, 30 May-1 June 2001.

Continuous affinity-based selection: rapid screening and simultaneous amplification of bacterial surface-display libraries

A new method for continuous biopanning has been developed. We have combined the power of affinity chromatography with the fecundity of bacteria in a unique process that mimics clonal selection. Mixed populations of bacteria were applied to a fermenter containing the immobilized ligand of interest. Bacteria retained in this affinity fermenter were allowed to grow under continuous washout conditions, such that weakly bound organisms were selectively lost. Those initially rare founder bacteria expressing a receptor for the immobilized ligand (R+ve) were thus enriched and amplified simultaneously. From an initial culture containing 1 x 10(10) R-ve cells spiked with fewer than 30 R+ve bacteria (<1 in 10(8)), final ratios of R+ve/R-ve bacteria as high as 1 in 12 were observed, representing an enrichment factor of 55 million-fold. This technology has considerable potential for rapid screening of bacterial surface-display libraries and in facilitating directed-evolution studies.

Unusually wide co-factor tolerance in a metalloenzyme; divalent metal ions modulate endo-exonuclease activity in T5 exonuclease

T5 5'-3' exonuclease is a member of a homologous group of 5' nucleases which require divalent metal co-factors. Structural and biochemical studies suggest that single-stranded DNA substrates thread through a helical arch or hole in the protein, thus bringing the phosphodiester backbone into close proximity with the active site metal co-factors. In addition to the expected use of Mg(2+), Mn(2+) and Co(2+) as co-factors, we found that divalent zinc, iron, nickel and copper ions also supported catalysis. Such a range of co-factor utilisation is unusual in a single enzyme. Some co-factors such as Mn(2+) stimulated the cleavage of double-stranded closed-circular plasmid DNA. Such endonucleolytic cleavage of circular double-stranded DNA cannot be readily explained by the threading model proposed for the cleavage of substrates with free 5'-ends as the hole observed in the crystal structure of T5 exonuclease is too small to permit the passage of double-stranded DNA. We suggest that such a substrate may gain access to the active site of the enzyme by a process which does not involve threading.

Variation in the steady state kinetic parameters of wild type and mutant T5 5'-3'-exonuclease with pH. Protonation of Lys-83 is critical for DNA binding

T5 5'-3'-exonuclease is a member of a family of homologous 5'-nucleases essential for DNA replication and repair. We have measured the variation of the steady state parameters of the enzyme with pH. The log of the association constant of the enzyme and substrate is pH-independent between pH 5 and 7, but at higher pH, it decreases (gradient -0.91 +/- 0.1) with increasing pH. The log of the turnover number increases (gradient 0.9 +/- 0.01) with increasing pH until a pH-independent plateau is reached. The T5 5'-3'-exonuclease-catalyzed reaction requires the protonation of a single residue for substrate binding, whereas kcat depends on a single deprotonation as demonstrated by the bell-shaped dependence of log (kcat/Km) on pH. To investigate the role of a conserved lysine (Lys-83), the pH profile of log (kcat/Km) of a K83A mutant was determined and found to increase with pH (gradient 1.01 +/- 0. 01) until a pH-independent plateau is reached. We therefore conclude that protonation of Lys-83 in the wild type protein facilitates DNA binding. The origin of the pH dependence of the kcat parameter of the wild type enzyme is discussed.

Invasive isolates of Neisseria meningitidis possess enhanced immunoglobulin A1 protease activity compared to colonizing strains

Neisseria meningitidis, Haemophilus influenzae, and Streptococcus pneumoniae possess the ability to cleave human IgA1 antibodies, and all successfully colonize and occasionally invade the human upper respiratory tract. N. meningitidis invades the bloodstream after a period of nasopharyngeal colonization. We directly compared levels of IgA1 protease activity in strains (n=52) derived from the cerebrospinal fluid or blood of patients with meningococcal disease with strains of N. meningitidis obtained from asymptomatic carriers (n=25). IgA1 protease activity was determined by a sensitive semiquantitative ELISA assay. Levels of IgA1 protease activity were significantly higher (P<0.0001) in strains associated with invasive meningococcal disease (98% with detectable activity, mean = 580 mU) than with those obtained from asymptomatic carriers (76% with detectable activity, mean = 280 mU). Despite marked variation in enzyme activity, almost all strains (96%) possessed the gene for IgA1 protease. Given the panmictic population structure of the bacterial isolates investigated, these data, obtained from two groups infected with N. meningitidis, but with markedly different clinical outcomes, provide the first quantitative evidence that IgA1 protease activity is a virulence determinant that contributes to the pathogenic phenotype, and suggest IgA1 protease as a potential target for prophylaxis.

A single cleavage assay for T5 5'-->3' exonuclease: determination of the catalytic parameters forwild-type and mutant proteins

Bacteriophage T5 5'-->3' exonuclease is a member of a family of sequence related 5'-nucleases which play an essential role in DNA replication. The 5'-nucleases have both exonucleolytic and structure-specific endo-nucleolytic DNA cleavage activity and are conserved in organisms as diverse as bacteriophage and mammals. Here, we report the development of a structure-specific single cleavage assay for this enzyme which uses a 5'-overhanging hairpin substrate. The products of DNA hydrolysis are characterised by mass spectrometry. The steady-state catalytic parameters of the enzyme are reported and it is concluded that T5 5'-->3' exonuclease accelerates the cleavage of a specific phosphodiester bond by a factor of at least 10(15). The catalytic assay has been extended to three mutants of T5 5'-->3' exonuclease, K83A, K196A and K215A. Mutation of any of these three lysine residues to alanine is detrimental to catalytic efficiency. All three lysines contribute to ground state binding of the substrate. In addition, K83 plays a significant role in the chemical reaction catalysed by this enzyme. Possible roles for mutated lysine residues are discussed.

Mutagenesis of conserved lysine residues in bacteriophage T5 5'-3' exonuclease suggests separate mechanisms of endo-and exonucleolytic cleavage

Efficient cellular DNA replication requires the activity of a 5'-3' exonuclease. These enzymes are able to hydrolyze DNA.DNA and RNA.DNA substrates exonucleolytically, and they are structure-specific endonucleases. The 5'-3' exonucleases are conserved in organisms as diverse as bacteriophage and mammals. Crystal structures of three representative enzymes identify two divalent-metal-binding sites typically separated by 8-10 A. Site-directed mutagenesis was used to investigate the roles of three lysine residues (K83, K196, and K215) situated near two metal-binding sites in bacteriophage T5 5'-3' exonuclease. Neither K196 nor K215 was essential for either the exo- or the endonuclease activity, but mutation of these residues increased the dissociation constant for the substrate from 5 nM to 200 nM (K196A) and 50 nM (K215A). Biochemical analysis demonstrated that K83 is absolutely required for exonucleolytic activity on single-stranded DNA but is not required for endonucleolytic cleavage of flap structures. Structural analysis of this mutant by x-ray crystallography showed no significant perturbations around the metal-binding sites in the active site. The wild-type protein has different pH optima for endonuclease and exonuclease activities. Taken together, these results suggest that different mechanisms for endo- and exonucleolytic hydrolysis are used by this multifunctional enzyme.

Structure-specific DNA cleavage by 5' nucleases

Many processes, such as DNA replication, recombination and repair, produce branched DNA structures. These bifurcated molecules are trimmed by a group of homologous 5'-3' exonucleases (also known as 5' nucleases) in structure-specific cleavage reactions. X-ray crystallographical and biochemical studies suggest that ssDNA substrates become threaded through the 5'-3' exonucleases, where hydrolysis is effected with the aid of divalent metal cations.

Prokaryotic 5'-3' exonucleases share a common core structure with gamma-delta resolvase

The three dimensional crystal structure of T5 5'-3' exonuclease was compared with that of two other members of the 5'-3' exonuclease family: T4 ribonuclease H and the N-terminal domain of Thermus aquaticus DNA polymerase I. Though these structures were largely similar, some regions of these enzymes show evidence of significant molecular flexibility. Previous sequence analysis had suggested the existence of a helix-hairpin-helix motif in T5 exonuclease, but a distinct, though related structure is actually found to occur. The entire T5 exonuclease structure was then compared with all the structures in the complete Protein Data Bank and an unexpected similarity with gamma-delta (gamma delta) resolvase was observed. 5'-3' exonucleases and gamma delta resolvase are enzymes involved in carrying out quite different manipulations on nucleic acids. They appear to be unrelated at the primary sequence level, yet the fold of the entire catalytic domain of gamma delta resolvase is contained within that of the 5'-3'exonuclease. Different large-scale helical structures are used by both families to form DNA binding sites.

Structure-specific DNA binding by bacteriophage T5 5'-->3' exonuclease

Phage T5 exonuclease is a 5'-->3'exodeoxyribonuclease that also exhibits endonucleolytic activity on flap structures (branched duplex DNA containing a free single-stranded 5'-end). Oligonucleotides were used to construct duplexes with either blunt ends, 5'-overhangs, 3'-overhangs, a flap or a forked end (pseudo-Y). The binding of T5 exonuclease to various structures was investigated using native electrophoretic mobility shift assays (EMSA) in the absence of the essential divalent metal cofactor. Binding of T5 exonuclease to either blunt-ended duplexes or single-stranded oligonucleotides could not be detected by EMSA. However, duplexes with 5'-overhangs, flaps and pseudo-Y structures showed decreased mobility with added T5 exonuclease. On binding to DNA the wild-type enzyme was rendered partially resistant to proteolysis, yielding a biologically active 31.5 kDa fragment. However, the protein-DNA complex remained susceptible to inactivation by p-hydroxymercuribenzoate (PHMB, a cysteine-specific modifying agent), suggesting that neither cysteine is intimately associated with substrate binding. Replacement of both cysteine residues of the molecule with serine did not greatly alter the catalytic or binding characteristics of the protein but did render it highly resistant to inhibition by PHMB.

Schistosoma mansoni: anomalous immunogenic properties of a 27 kDa larval serine protease associated with protective immunity

A cationic Schistosoma mansoni cercarial antigen was shown to be a serine protease as it was capable of hydrolysing N-acetyl-DL-phenylalanine beta-naphthyl ester (NAPBNE) after precipitation by immunoelectrophoresis, and this reaction was modulated by the serine protease inhibitors phenylmethanesulfonyl fluoride (PMSF) and diisopropylfluorophosphate (DEP). The antigen in the immunoprecipitin arcs could also be radio-isotope labelled with tritiated DFP. The peptidolytic enzyme identified in immunoelectrophoresis with polyspecific sera and radio-isotope labelled with tritiated DFP had a relative molecular size of approximately 27 kDa in sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), and evidence obtained after partial purification, SDS-PAGE and immunoblotting supported this size estimate for the enzyme. A rabbit antiserum raised against the peptidolytic antigen reacted against a doublet of antigens at 27/28 kDa in immunoelectrophoresis arcs and against an antigen of 60 kDa in Western immunoblots of crude cercarial homogenate. However, the latter serum precipitated the cationic antigen in immunoelectrophoresed cercarial homogenates only after pre-incubation of the homogenates with PMSF. Fractions containing the partially purified protease also degraded radio-isotope labelled human IgG. The reactivity of a range of polyspecific and monospecific rabbit antisera in Western blots with larval extracts indicated that antibody responses against the 27/28 kDa doublet may be modulated. When immunized with material which contained the 27 kDa enzyme as a major constituent, and which was secreted by S. mansoni cercariae during transformation, only 5 of 16 mice produced antibody to this antigen that was detectable in Western blots. The 5 antibody 'responder' mice were significantly (P < 0.001) protected against challenge with a percutaneous infection of S. mansoni cercariae compared with a group of a mice also immunized with CTF, but which had not produced antibodies against the 27/28 kDa doublet. The results indicate that the 27 kDa serine protease of S. mansoni larvae is a target that is sensitive to immunological attack.

Cloning, heterologous expression and antigenicity of a schistosome cercarial protease

A gene coding for the 30 kDa Schistosoma mansoni cercarial protease was amplified using the polymerase chain reaction (PCR) from genomic DNA templates. Cloning and sequencing of several independent PCR clones revealed the presence of an intron additional to the one described in the original cloning of the gene. The 3 exons were cloned into expression vectors so that they could be expressed as separate glutathione-S-transferase (GST) translational fusions. Recombinant bacteria carrying these expression plasmids expressed the fusion proteins at high levels. Western blotting of bacterial lysates with sera raised against the native S. mansoni cercarial protease showed that all 3 exons were recognized. Thus we have produced recombinant bacteria capable of providing large amounts of an S. mansoni antigen for immunological studies and evaluation as a candidate vaccine.

Identification and eradication of a denatured DNA isolated during alkaline lysis-based plasmid purification procedures

Many plasmid isolation procedures use strongly alkaline conditions in the initial stages to facilitate lysis of the host bacteria. We demonstrate that such procedures can give rise to a minor but significantly altered form of plasmid. After electrophoresis and uv transillumination of ethidium bromide-stained agarose gels we and others have noticed a faint band migrating near to the major fluorescent product, covalently closed circular plasmid DNA. This faint band is resistant to cleavage by restriction endonucleases which have recognition sites in the parent plasmid. We were able to show that the contaminating band is able to transform competent Escherichia coli cells and that normal double-stranded plasmids were isolated from such transformants. We were able to selectively hydrolyze the contaminating band using T5 exonuclease which is a 5'-nuclease with a single-strand specific endonuclease activity. Plasmid preparations carried out under nonalkaline conditions failed to produce the contaminant band. We suggest methods for purifying plasmid DNA which remove the denatured band and could improve cloning efficiencies where the largest recombinant libraries are required.

A helical arch allowing single-stranded DNA to thread through T5 5'-exonuclease

THE 5'-exonucleases are enzymes that are essential for DNA replication and repair. As well as their exonucleolytic action, removing nucleotides from the 5'-end of nucleic acid molecules such as Okazaki fragments, many 5'-3'-exonucleases have been shown to possess endonucleolytic activities. T5 5'-3'-exonuclease shares many similarities with the amino terminal of eubacterial DNA polymerases, although, unlike eubacteria, phages such as T5, T4 and T7 express polymerase and 5'-exonuclease proteins from separate genes. Here we report the 2.5-A crystal structure of the phage T5 5'-exonuclease, which reveals a helical arch for binding DNA. We propose a model consistent with a threading mechanism in which single-stranded DNA could slide through the arch, which is formed by two helices, one containing positively charged, and the other hydrophobic, residues. The active site is at the base of the arch, and contains two metal-binding sites.

The evolution of pathways for aromatic hydrocarbon oxidation in Pseudomonas

The organisation and nucleotide sequences coding for the catabolism of benzene, toluene (and xylenes), naphthalene and biphenyl via catechol and the extradiol (meta) cleavage pathway in Pseudomonas are reviewed and the various factors which may have played a part in their evolution are considered. The data suggests that the complete pathways have evolved in a modular way probably from at least three elements. The common meta pathway operons, downstream from the ferredoxin-like protein adjacent to the gene for catechol 2,3-dioxygenase, are highly homologous and clearly share a common ancestry. This common module may have become fused to a gene or genes the product(s) of which could convert a stable chemical (benzoate, salicylate, toluene, benzene, phenol) to catechol, thus forming the lower pathway operons found in modern strains. The upper pathway operons might then have been acquired as a third module at a later stage thus increasing the catabolic versatility of the host strains.

Computer aided identification of a potential 5'-3' exonuclease gene encoded by Escherichia coli

The predicted amino acid sequence of a previously overlooked open reading frame (ORF) from E. coli has been subjected to sequence analysis. The ORF, sequenced independently by two research groups, appears to encode a protein of 251 amino acids residues (25 kDa). The protein sequence shares a high degree of similarity with the amino acid termini of E. coli DNA polymerase I and other bacterial DNA polymerases (and phage-encoded enzymes) known to possess 5'-3' exonuclease activity. The newly identified E. coli gene, designated exo, is positioned downstream of signals characteristic of an efficient translation initiation sequence. Codon bias analysis indicates that the ORF has good protein coding potential. These observations indicate that the gene is likely to be expressed. The gene is 60% identical with the 5'-region of the E. coli DNA PolI gene (polA) over a 260 base-pair region. These two genes may have arisen by duplication of an ancestral gene or polA may have arisen by a recombination event involving exo. A comparison of the predicted secondary structures of 5'-3' exonucleases revealed the presence of several conserved regions of secondary structure, including a potential helix-turn-helix motif.

Two identical copies of IS1246, a 1275 base pair sequence related to other bacterial insertion sequences, enclose the xyl genes on TOL plasmid pWW0

Two identical direct repeats of a 1275 bp sequence, designated IS1246, encompass the xyl genes, which determine the catabolism of toluene, m- and p-xylenes to central metabolites, on the TOL catabolic plasmid pWW0. IS1246 has a terminal inverted repeat of 12 bp (5'GGGCACCTCGAA3') and contains a major open reading frame of 280 codons. This ORF shows significant homology with ORFs encoded by a number of bacterial insertion sequences from Bacteroides, Neisseria and Escherichia coli.

Preliminary crystallographic studies on the D15 5' to 3' exonuclease from phage T5

The D15 exonuclease from phage T5 has been crystallized from 35% (w/v) ammonium sulfate by the hanging drop vapor diffusion technique. The crystals grow in tetragonal space group P4(1)22 or P4(3)22 with cell dimensions a = b = 79.2 A and c = 138.0 A. The crystals diffract to 2.5 A and are suitable for X-ray structure determination.

Rapid high-efficiency site-directed mutagenesis by the phosphorothioate approach

Several improvements to the existing phosphorothioate-based site-directed mutagenesis methodology are reported, and here it is demonstrated that the new procedure is able to produce large deletions, insertions and point mutations rapidly and with very high efficiency. The time required for the polymerization step has been reduced by using T7 DNA polymerase to extend the mutant oligonucleotide primer-template. The reaction produces good yields of double-stranded closed-circular DNA and some partially polymerized template. The reaction was treated with T5 D15 exonuclease to selectively destroy partially polymerized single-stranded phage DNA that may otherwise contribute to an increased background of wild-type transformants. The use of these enzymes greatly facilitates the implementation of the phosphorothioate-based site-directed mutagenesis method by requiring less template DNA and by allowing all the in vitro manipulations to be completed in a day. In its present form the method may easily be automated, enabling large systematic site-directed mutagenesis projects to be undertaken.

A single-strand specific endonuclease activity copurifies with overexpressed T5 D15 exonuclease

The T5 D15 exonuclease purified from an overproducing strain of E. coli was shown to possess a low level of endonucleolytic activity specific for single-stranded DNA when assayed with 1-10 mM Mg2+ as co-factor. Endonuclease activity on double-stranded circular DNA could not be detected under these conditions. Nicked circular DNA was first gapped by the enzyme's exonucleolytic activity, creating a single-stranded region. This gapped substrate was then endonucleolytically cleaved and rapidly degraded. We show that a gapped and not a nicked substrate is required for this activity as previously suggested (Moyer, R. W. and Roth, C. T. 1977, J. Virol. 24, 177-193). The single-strand endonuclease activity could be selectively suppressed by using low concentrations of Mg2+ as co-factor (less than 1 mM), thus allowing nicked double-stranded circular DNA to be gapped to a single-stranded circular species. We also report on sequence similarities between the T5 exonuclease and several prokaryotic DNA polymerases.

Properties of overexpressed phage T5 D15 exonuclease. Similarities with Escherichia coli DNA polymerase I 5'-3' exonuclease

The D15 gene of the bacteriophage T5, thought to encode an exonuclease, was cloned into an M13 phage on a 1344-base pair fragment. The deduced amino acid sequence of 291 residues (Kaliman, A. V., Krutilina, A. I., Kryukov, V. M., and Bayev, A. A. (1986) FEBS Lett. 195, 61-64) shows a high degree of homology with the first 320 amino acid residues of Escherichia coli DNA polymerase I, the region containing the enzyme's 5'-3' exonuclease activity. Recombinant M13 phage DNA was manipulated by oligonucleotide-directed mutagenesis to enable subcloning into a high efficiency expression vector, allowing the production of large amounts of enzyme for physical characterization and crystallization trials. The enzyme was purified to homogeneity. The purified enzyme is active on both native and heat-denatured DNA and shows no endonuclease activity on either double-stranded closed-circular or nicked DNA. The enzyme is also able to degrade some oligonucleotides in a manner which depends not only on the nucleotide sequence but also on the state of hybridization of the potential substrate. The mode of action of this enzyme is similar to, although not identical to that of the 5'-3' exonuclease activity of E. coli DNA polymerase I.

Inhibition of the restriction endonuclease BanII using modified DNA substrates. Determination of phosphate residues critical for the formation of an active enzyme-DNA complex

The restriction endonuclease BanII catalyzes the cleavage of double-stranded DNA and recognizes the degenerate sequence 5'-GPuGCPyC-3'. The poly-linker of M13mp18 contains one such sequence, 5'-GAGCTC-3'. The three other possible sites recognized by the enzyme were prepared by site-directed muta-genesis. The substitution of phosphate groups by phosphorothioate residues at some positions within the various recognition sites had relatively little effect on the rate of cleavage of the DNA. However, when the DNA contained a phosphorothioate group at the site of cleavage the rate of linearization of the DNA was decreased by a factor of 9. Interestingly, DNA which contained an additional phosphorothioate internucleotidic linkage immediately 3'-outside the recognition site could not be linearized by the enzyme. The results indicate that an important contact between enzyme and substrate is perturbed by the presence of the sulfur atom at this position.

5'-3' exonucleases in phosphorothioate-based oligonucleotide-directed mutagenesis

The application of T7 and lambda exonuclease to phosphorothioate-based oligonucleotide-directed mutagenesis was investigated. Oligonucleotide primers designed to introduce single or double base mismatches, an insertion or a deletion (each of 16 bases) were annealed to M13 phage derivatives. Double stranded closed circular DNA (RF IV) containing phosphorothioate internucleotidic linkages in the (-)strand was prepared enzymatically from these templates. A nick was introduced into the (+)strand of the hetroduplex DNA. This nicked DNA (RF II) was subjected to treatment with T7 or lambda exonuclease. Both of these enzymes were able to degrade almost all of the viral (+)strand when presented with DNA containing one or two base mismatches. Repolymerisation of the DNA after the gapping reaction, followed by transfection into E. coli cells gave mutational efficiencies of up to 95%. In the case of RF II DNA prepared with insertion or deletion primers these exonucleases could only partially degrade the viral (+)strand but were nevertheless highly efficient in such mutagenesis experiments.

Strand specific cleavage of phosphorothioate-containing DNA by reaction with restriction endonucleases in the presence of ethidium bromide

A method for achieving strand specific nicking of DNA has been developed. Phosphorothioate groups were incorporated enzymatically into the (-)strand of M13 RF IV DNA. When such DNA is reacted with restriction endonucleases in the presence of ethidium bromide nicked DNA (RF II) is produced. All of the restriction enzymes tested linearised phosphorothioate-containing DNA in the absence of this dye. The strand specificity of the reaction was investigated by employing the ethidium bromide mediated nicking reaction in the phosphorothioate-based oligonucleotide-directed mutagenesis method. The mutational efficiencies obtained were in the region of 64-89%, indicating that these restriction enzymes hydrolyse the phosphodiester bond at the cleavage site of the unsubstituted (+)strand.

Synthesis and antiviral properties of (E)-5-(2-bromovinyl)-2'-deoxycytidine-related compounds

Treatment of 3',5'-di-O-acetyl-(E)-5-(2-bromovinyl)-2'-deoxyuridine (2) with p-chlorophenyl phosphorodichloridate and 1,2,4-triazole gave 1-(3,5-di-O-acetyl-2-deoxy-beta-D-erythro-pentofuranosyl)-(E)-5-(2-br o movinyl)- 4-(1,2,4-triazol-1-yl)pyrimidin-2(1H)-one (3). Reaction of 3 with ammonia gave (E)-5-(2-bromovinyl)-2'-deoxycytidine (1), the overall yield from 2 being 60%. A similar 4-(1,2,4-triazol-1-yl) derivative (4) was obtained from 3',5'-di-O-acetyl-thymidine by the use of phosphoryl chloride as the condensing agent. Treatment of thymidine with trimethylsilyl chloride and then with phosphoryl chloride and 1,2,4-triazole gave upon workup 1-(2-deoxy-beta-D-erythro-pentofuranosyl)-5-methyl-4(1,2,4-triazol -1-yl) pyrimidin-2(1H)-one (5). (E)-5-(2-Bromovinyl)-2'-deoxyuridine (BVDU) when similarly treated gave the corresponding (E)-5-(2-bromovinyl) compound 7. A minor product formed in both cases was a 4-(1,2,4-triazol-1-yl) derivative in which the nucleoside 5'-hydroxyl group had been replaced by chlorine (6 and 8). Whereas compounds 4-6 and 8 did not exhibit a selective antiviral effect, compounds 1-3 and 7 proved almost as active as the reference compound BVDU. In particular, compound 7, the 4-triazolyl derivative of BVDU, would seem worth pursuing for its potential as an inhibitor of herpes simplex virus type 1 and varicella-zoster virus.