Tumour cell retention of rucaparib, sustained PARP inhibition and efficacy of weekly as well as daily schedules

Background:

Poly(ADP-ribose) polymerase-1 (PARP) inhibitors (PARPi) exploit tumour-specific defects in homologous recombination DNA repair and continuous dosing is most efficacious. Early clinical trial data with rucaparib suggested that it caused sustained PARP inhibition. Here we investigate the mechanism of this durable inhibition and potential exploitation.

Methods:

Uptake and retention of rucaparib and persistence of PARP inhibition were determined by radiochemical and immunological assays in human cancer cell lines. The pharmacokinetics and pharmacodynamics of rucaparib were determined in tumour-bearing mice and the efficacy of different schedules of rucaparib was determined in mice bearing homologous recombination DNA repair-defective tumours.

Results:

Rucaparib accumulation is carrier mediated (Km=8.4±1.2 μM, Vmax=469±22 pmol per 10⁶ cells per 10 min), reaching steady-state levels >10 times higher than the extracellular concentration within 30 min. Rucaparib is retained in cells and inhibits PARP ⩾50% for ⩾72 h days after a 30-min pulse of 400 nM. In Capan-1 tumour-bearing mice rucaparib accumulated and was retained in the tumours, and PARP was inhibited for 7 days following a single dose of 10 mg kg⁻¹ i.p or 150 mg kg⁻¹ p.o. by 70% and 90%, respectively. Weekly dosing of 150 mg kg⁻¹ p.o once a week was as effective as 10 mg kg⁻¹ i.p daily for five days every week for 6 weeks in delaying Capan-1 tumour growth.

Conclusions:

Rucaparib accumulates and is retained in tumour cells and inhibits PARP for long periods such that weekly schedules have equivalent anticancer activity to daily dosing in a pre-clinical model, suggesting that clinical evaluation of alternative schedules of rucaparib should be considered.

Resistance-modifying agents. 9. Synthesis and biological properties of benzimidazole inhibitors of the DNA repair enzyme poly(ADP-ribose) polymerase

The nuclear enzyme poly(ADP-ribose) polymerase (PARP) facilitates the repair of DNA strand breaks and is implicated in the resistance of cancer cells to certain DNA-damaging agents. Inhibitors of PARP have clinical potential as resistance-modifying agents capable of potentiating radiotherapy and the cytotoxicity of some forms of cancer chemotherapy. The preclinical development of 2-aryl-1H-benzimidazole-4-carboxamides as resistance-modifying agents in cancer chemotherapy is described. 1H-Benzimidazole-4-carboxamides, particularly 2-aryl derivatives, are identified as a class of potent PARP inhibitors. Derivatives of 2-phenyl-1H-benzimidazole-4-carboxamide (23, K(i) = 15 nM), in which the phenyl ring contains substituents, have been synthesized. Many of these derivatives exhibit K(i) values for PARP inhibition < 10 nM, with 2-(4-hydroxymethylphenyl)-1H-benzimidazole-4-carboxamide (78, K(i) = 1.6 nM) being one of the most potent. Insight into structure-activity relationships (SAR) for 2-aryl-1H-benzimidazole-4-carboxamides has been enhanced by studying the complex formed between 2-(3-methoxyphenyl)-1H-benzimidazole-4-carboxamide (44, K(i) = 6 nM) and the catalytic domain of chicken PARP. Important hydrogen-bonding and hydrophobic interactions with the protein have been identified for this inhibitor. 2-(4-Hydroxyphenyl)-1H-benzimidazole-4-carboxamide (45, K(i) = 6 nM) potentiates the cytotoxicity of both temozolomide and topotecan against A2780 cells in vitro (by 2.8- and 2.9-fold, respectively).

Potentiation of temozolomide and topotecan growth inhibition and cytotoxicity by novel poly(adenosine diphosphoribose) polymerase inhibitors in a panel of human tumor cell lines

Potent poly(ADP-ribose) polymerase (PARP) inhibitors have been developed that potentiate the cytotoxicity of ionizing radiation and anticancer drugs. The biological effects of two novel PARP inhibitors, NU1025 (8-hydroxy-2-methylquinazolin-4-[3H]one, Ki = 48 nM) and NU1085 [2-(4-hydroxyphenyl)benzamidazole-4-carboxamide, Ki = 6 nM], in combination with temozolomide (TM) or topotecan (TP) have been studied in 12 human tumor cell lines (lung, colon, ovary, and breast cancer). Cells were treated with increasing concentrations of TM or TP +/- NU1025 (50, 200 microM) or NU1085 (10 microM) for 72 h. The potentiation of growth inhibition by NU1025 and NU1085 varied between the cell lines from 1.5- to 4-fold for TM and 1- to 5-fold for TP and was unaffected by p53 status. Clonogenic assays undertaken in two of the cell lines confirmed that the potentiation of growth inhibition reflected the potentiation of cytotoxicity. NU1025 (50 microM) was about as effective as 10 microM NU1085 at potentiating growth inhibition and cytotoxicity, consistent with the relative potencies of the two molecules as PARP inhibitors. Potentiation of cytotoxicity was obtained at concentrations of NU1025 and NU1085 that were not toxic per se; however, NU1085 alone was 3-fold more cytotoxic (LC50 values ranged from 83 to 94 microM) than NU1025 alone (LC50 > 900 microM). These data demonstrate that PARP inhibitors are effective resistance-modifying agents in human tumor cell lines and have provided a comprehensive assessment protocol for the selection of optimum combinations of anticancer drugs, PARP inhibitors, and cell lines for in vivo studies.

Base excision repair is impaired in mammalian cells lacking Poly(ADP-ribose) polymerase-1

In mammalian cells, damaged bases in DNA are corrected by the base excision repair pathway which is divided into two distinct pathways depending on the length of the resynthesized patch, replacement of one nucleotide for short-patch repair, and resynthesis of several nucleotides for long-patch repair. The involvement of poly(ADP-ribose) polymerase-1 (PARP-1) in both pathways has been investigated by using PARP-1-deficient cell extracts to repair single abasic sites derived from uracil or 8-oxoguanine located in a double-stranded circular plasmid. For both lesions, PARP-1-deficient cell extracts were about half as efficient as wild-type cells at the polymerization step of the short-patch repair synthesis, but were highly inefficient at the long-patch repair. We provided evidence that PARP-1 constitutively interacts with DNA polymerase beta. Using cell-free extracts from mouse embryonic cells deficient in DNA polymerase beta, we demonstrated that DNA polymerase beta is involved in the repair of uracil-derived AP sites via both the short and the long-patch repair pathways. When both PARP-1 and DNA polymerase beta were absent, the two repair pathways were dramatically affected, indicating that base excision repair was highly inefficient. These results show that PARP-1 is an active player in DNA base excision repair.

The conformation of hepatitis C virus NS3 proteinase with and without NS4A: a structural basis for the activation of the enzyme by its cofactor

BACKGROUND

Hepatitis C virus (HCV) NS3 proteinase activity is required for the release of HCV nonstructural proteins and is thus a potential antiviral target. The enzyme requires a protein cofactor NS4A, located downstream of NS3 on the polyprotein, for activation and efficient processing.

OBJECTIVES

Comparison of the proteinase three-dimensional structure before and after NS4A binding should help to elucidate the mechanism of NS4A-dependent enzyme activation.

STUDY DESIGN

We determined the crystal structure of NS3 proteinase of HCV BK isolate (genotype 1b; residues 1-189) and also the crystal structure of this proteinase complexed with HCV BK-NS4A (residues 21-34).

RESULTS

The core region (residues 30-178) of the enzyme without cofactor (NS3P) or with bound cofactor (NS3P/4A) is folded into a trypsin-like conformation and the substrate P1 specificity pocket is essentially unchanged. However, the D1-E1 beta-loop shifts away from the cofactor binding site in NS3P/4A relative to NS3P, thereby accommodating NS4A. One result is that catalytic residues His-57 and Asp-81 move closer to Ser-139 and their sidechains adopt more 'traditional' (trypsin-like) orientation. The N-terminus (residues 1-30), while extended in NS3P, is folded into an alpha-helix and beta-strand that cover the bound cofactor of NS3P/4A. A new substrate-binding surface is formed from both the refolded N-terminus and NS4A, potentially affecting substrate residues immediately downstream of the cleavage site.

CONCLUSIONS

Direct comparison of the crystal structures of NS3P and NS3P/4A shows that the binding of NS4A improves the anchoring and orientation of the enzyme's catalytic triad. This is consistent with the enhancement of NS3P's weak residual activity upon NS4A binding. There is also significant refolding of the enzyme's N-terminus which provides new interactions with P'-side substrate residues. The binding surface for P'-side substrate residues, including the P1 specificity pocket, changes little after NS4A binding. In summary, we observe a structural basis for improved substrate turnover and affinity that follows complexation of NS3P with its NS4A cofactor.

Dicaffeoylquinic and dicaffeoyltartaric acids are selective inhibitors of human immunodeficiency virus type 1 integrase

Current pharmacological agents for human immunodeficiency virus (HIV) infection include drugs targeted against HIV reverse transcriptase and HIV protease. An understudied therapeutic target is HIV integrase, an essential enzyme that mediates integration of the HIV genome into the host chromosome. The dicaffeoylquinic acids (DCQAs) and the dicaffeoyltartaric acids (DCTAs) have potent activity against HIV integrase in vitro and prevent HIV replication in tissue culture. However, their specificity against HIV integrase in cell culture has been questioned. Thus, the ability of the DCQAs and DCTAs to inhibit binding of HIV type 1 (HIV-1) gp120 to CD4 and their activities against HIV-1 reverse transcriptase and HIV RNase H were studied. The DCQAs and DCTAs inhibited HIV-1 integrase at concentrations between 150 and 840 nM. They inhibited HIV replication at concentrations between 2 and 12 microM. Their activity against reverse transcriptase ranged from 7 microM to greater than 100 microM. Concentrations that inhibited gp120 binding to CD4 exceeded 80 microM. None of the compounds blocked HIV-1 RNase H by 50% at concentrations exceeding 80 microM. Furthermore, when the effects of the DCTAs on reverse transcription in acutely infected cells were measured, they were found to have no activity. Therefore, the DCQAs and DCTAs exhibit > 10- to > 100-fold specificity for HIV integrase, and their activity against integrase in biochemical assays is consistent with their observed anti-HIV activity in tissue culture. Thus, the DCQAs and DCTAs are a potentially important class of HIV inhibitors that act at a site distinct from that of current HIV therapeutic agents.

The NS3 proteinase domain of hepatitis C virus is a zinc-containing enzyme

NS3 proteinase of hepatitis C virus (HCV), contained within the N-terminal domain of the NS3 protein, is a chymotrypsin-like serine proteinase responsible for processing of the nonstructural region of the HCV polyprotein. In this study, we examined the sensitivity of the NS3 proteinase to divalent metal ions, which is unusual behavior for this proteinase class. By using a cell-free coupled transcription-translation system, we found that HCV polyprotein processing can be activated by Zn2+ (and, to a lesser degree, by Cd2+, Pb2+, and Co2+) and inhibited by Cu2+ and Hg2+ ions. Elemental analysis of the purified NS3 proteinase domain revealed the presence of zinc in an equimolar ratio. The zinc content was unchanged in a mutated NS3 proteinase in which active-site residues His-57 and Ser-139 were replaced with Ala, suggesting that the zinc atom is not directly involved in catalysis but rather may have a structural role. Based on data from site-directed mutagenesis combined with zinc content determination, we propose that Cys-97, Cys-99, Cys-145, and His-149 coordinate the structural zinc in the HCV NS3 proteinase. A similar metal binding motif is found in 2A proteinases of enteroviruses and rhinoviruses, suggesting that these 2A proteinases and HCV NS3 proteinase are structurally related.

A specific RNA structural motif mediates high affinity binding by the HIV-1 nucleocapsid protein (NCp7)

Current research indicates that the nucleocapsid protein (NCp7) of human immunodeficiency virus type 1 (HIV-1) interacts with a variety of RNA substrates during the progression of the viral life cycle. The RNA features specifically recognized by the protein, however, have yet to be identified. SELEX was used to generate a set of RNAs whose affinities for nucleocapsid were on the order of 2 x 10(-9) M. Comparative analysis revealed that each RNA contains a highly conserved fourteen nucleotide sequence-block. Computer modeling and structure probing experiments indicate that the RNA ligands use the consensus sequence to fold into hairpins with an identical asymmetric bulge. The presence of the nucleocapsid protein protects the asymmetric bulge from ribonuclease attack, suggesting that it is the key element in protein recognition. A search for similar structural motifs within the HIV genome reveals several potential interaction sites for the nucleocapsid protein.

The crystal structure of hepatitis C virus NS3 proteinase reveals a trypsin-like fold and a structural zinc binding site

During replication of hepatitis C virus (HCV), the final steps of polyprotein processing are performed by a viral proteinase located in the N-terminal one-third of nonstructural protein 3. The structure of NS3 proteinase from HCV BK strain was determined by X-ray crystallography at 2.4 angstrom resolution. NS3P folds as a trypsin-like proteinase with two beta barrels and a catalytic triad of His-57, Asp-81, Ser-139. The structure has a substrate-binding site consistent with the cleavage specificity of the enzyme. Novel features include a structural zinc-binding site and a long N-terminus that interacts with neighboring molecules by binding to a hydrophobic surface patch.

Structure-function analysis of the mammalian DNA polymerase beta active site: role of aspartic acid 256, arginine 254, and arginine 258 in nucleotidyl transfer

The crystal structure of the catalytic domain of rat DNA polymerase beta revealed that Asp256 is located in proximity to the previously identified active site residues Asp190 and Asp192. We have prepared and kinetically characterized the nucleotidyl transfer activity of wild type and several mutant forms of human and rat pol beta. Herein we report steady-state kinetic determinations of KmdTTP, Km(dT)16, and kcat for mutants in residue Asp256 and two neighboring residues, Arg254 and Arg258, all centrally located on strand beta 7 in the pol beta structure. Mutation of Asp256 to alanine abolished the enzymatic activity of pol beta. Conservative replacement with glutamic acid (D256E) led to a 320-fold reduction of kcat compared to wild type. Replacement of Arg254 with an alanine (R254A) resulted in a 50-fold reduction of kcat compared to wild type. The Km(dT)16 of D256E and R254A increased about 18-fold relative to wild type. Replacement of Arg254 with a lysine resulted in a 15-fold decrease in kcat, and a 5-fold increase in the Km(dT)16. These kinetic observations support a role of Asp256 and Arg254 in the positioning of divalent metal ions and substrates in precise geometrical orientation for efficient catalysis. The mutation of Arg258 to alanine (R258A) resulted in a 10-fold increase in KmdTTP and a 65-fold increase in Km(dT)16 but resulted in no change of kcat. These observations are discussed in the context of the three-dimensional structures of the catalytic domain of pol beta and the ternary complex of pol beta, ddCTP, and DNA.

High-affinity ssDNA inhibitors of the reverse transcriptase of type 1 human immunodeficiency virus

The reverse transcriptase (RT) of HIV-1 is a plausible target for therapeutic agents aimed at inhibiting propagation of the virus. We have used "irrational drug design", that is, combinatorial chemistry with oligonucleotide libraries, to identify high-affinity ligands aimed at HIV-1 RT. The methodology, termed SELEX (systematic evolution of ligands by exponential enrichment), was employed with a single-stranded DNA library. The selected ssDNA ligands bind HIV-1 RT with Kd values as low as 1 nM and inhibit the RNA-dependent DNA-polymerase activity of the enzyme with Ki values as low as 0.3 nM. We also demonstrate the high specificity of one ligand able to selectively discriminate between the reverse transcriptases of HIV-1, AMV, and MMLV. These ssDNA molecules may be useful as inhibitors or as models for the design of small molecule inhibitors of HIV-1 RT in vivo.

Comprehensive chemical modification interference and nucleotide substitution analysis of an RNA pseudoknot inhibitor to HIV-1 reverse transcriptase

We had previously used in vitro RNA selection techniques to describe a consensus RNA pseudoknot that binds and inhibits HIV-1 reverse transcriptase (HIV-RT). In this work we constructed variants of this consensus pseudoknot in order to evaluate the contributions of individual nucleotide identities and secondary structure to affinity for HIV-RT. We have also used chemical modification of ligand RNAs to corroborate the predicted structure of the pseudoknot, to discover which modifiable groups are protected from chemical attack when bound to HIV-RT, and to find which modifications interfere with binding to HIV-RT. A novel interference study is presented which involves selection of ligands from a pool created by mixed reagent oligonucleotide synthesis in order to rapidly determine allowed substitutions of 2'-OCH3 groups for the usual 2'-OH group in such RNA ligands.

Restriction of HIV-1 replication in intestinal cells is genetically controlled by the gag-pol region of the HIV-1 genome

The human colon epithelial line HT29 represents a semipermisive cellular system for human immunodeficiency virus type 1 (HIV-1). It could be productively infected with HIV-1 NDK, a Zairian virus isolate highly cytopathic for CD4 positive lymphocytes, whereas infection with the prototype virus HIV-1 LAV was nonproductive. Recombinant viruses derived from HIV-1 LAV and HIV-1 NDK were used to determine the genetic control, step of virus/cell cycle, and molecular mechanism responsible for productive versus nonproductive infection of intestinal cells. Both parental viruses and all recombinants retrotranscribed their genomes with a similar kinetics and were able to complete HIV-1 DNA synthesis, HIV-1 LAV provirus present in preintegration complexes could be rescued by cocultivation with T-lymphocytes. However, it was aborted during prolonged cultivation of HT29 cells. Our results suggest that (i) gag/pol region of HIV-1 genome (fragment BssHII255-EcoRI4183) genetically controlled productive infection of intestinal cells and that (ii) the difference between productive and abortive infection occurred before synthesis of HIV-1 mRNA, at the integration level.

Structures of DNA and RNA polymerases and their interactions with nucleic acid substrates

DNA and RNA polymerases are enzymes that are primarily responsible for copying genetic material in all living systems. The four polymerases whose structures have been determined by X-ray crystallographic methods have significant similarities at the polymerase active site that are indicative of common requirements for polynucleotide synthesis. Structural studies of complexes of the Klenow fragment of Escherichia coli DNA polymerase I, HIV type 1 reverse transcriptase, and rat DNA polymerase beta with DNA are leading to generalized models for catalysis.

Subunit composition and domain structure of the Spo0A sporulation transcription factor of Bacillus subtilis

The Spo0A transcription factor is responsible for the initiation of sporulation and is active in transcription only after phosphorylation by a specific signal transduction pathway, the phosphorelay. The effect of phosphorylation on the physical properties of Spo0A was determined. Spo0A and Spo0A approximately P both behaved as monomers during Sephacryl chromatography and gel electrophoresis, suggesting that phosphorylation did not modify the oligomerization state of the protein. Trypsin digested Spo0A at a single cleavage site between residues 142 and 143 within a hinge connecting two tightly folded domains. The amino domain retains ability to be phosphorylated by the phosphorelay. The carboxyl domain is active as a DNA-binding protein and retains the sequence specificity of the intact molecule for 0A boxes on the abrB promoter as revealed by footprinting studies. The carboxyl domain stimulated in vitro transcription from the spoIIG promoter 5-fold greater than an equal amount of Spo0A and about half as well as equivalent amounts of Spo0A approximately P. Thus, the unphosphorylated amino domain inhibits the transcription stimulation activity of the carboxyl domain. We suggest that phosphorylation activates transcription regulation functions of Spo0A by modifying the spatial relationships of the amino and carboxyl domains.

2.3 A crystal structure of the catalytic domain of DNA polymerase beta

The crystal structure of the catalytic domain of rat DNA polymerase beta (pol beta) has been determined at 2.3 A resolution and refined to an R factor of 0.22. The mixed alpha/beta protein has three subdomains arranged in an overall U shape reminiscent of other polymerase structures. The folding topology of pol beta, however, is unique. Two divalent metals bind near three aspartic acid residues implicated in the catalytic activity. In the presence of Mn2+ and dTTP, interpretable electron density is seen for two metals and the triphosphate, but not the deoxythymidine moiety. The principal interaction of the triphosphate moiety is with the bound divalent metals.

Redesignation of the RNase D activity associated with retroviral reverse transcriptase as RNase H

In the presence of Mn2+, reverse transcriptase of both human immunodeficiency virus and murine leukemia virus hydrolyzes duplex RNA. However, designating this novel activity RNase D conflicts with Escherichia coli RNase D, which participates in tRNA processing. On the basis of its location in the RNase H domain, we propose that this novel retroviral activity be redesignated RNase H*.

RNase D, a reported new activity associated with HIV-1 reverse transcriptase, displays the same cleavage specificity as Escherichia coli RNase III

RNase D was recently reported as a new enzymatic activity associated with HIV-1 reverse transcriptase (RT), cleaving RNA at two positions within the double-stranded region of the tRNA primer-viral RNA template complex (Ben-Artzi et al., Proc. Natl. Acad. Sci. USA 89 (1992) 927-931). This would make RNase D a fourth distinct activity of HIV-1 RT, in addition to RNA- and DNA-dependent DNA polymerase and RNase H. Using a specific substrate containing tRNA(Lys,3) hybridized to the primer binding site, we were able to detect the reported RNase D activity in our preparations of recombinant HIV-1 RT. This activity was also present in several active-site mutants of RT, suggesting that it is independent of the RNase H and polymerase functionalities of RT. Furthermore, we found that the cleavage specificity of RNase D is the same as that of RNase III isolated from E.coli. A likely explantation of these results--that the observed RNase D activity is attributable to traces of RNase III contamination--was further strengthened by the finding that the recombinant preparations of HIV-1 RT can specifically cleave a phage T7-derived double-stranded RNA processing signal, which has been used as a model substrate for detection of E.coli RNase III. Moreover, RT purified from an RNase III- strain of E.coli displayed no cleavage of the tRNA primer-RNA template complex.

Reverse transcriptase of human immunodeficiency virus type 1: functionality of subunits of the heterodimer in DNA synthesis

From an in vitro analysis of the DNA-synthesizing abilities of certain specifically mutated forms of the heterodimeric reverse transcriptase of human immunodeficiency virus type 1, we can conclude that in a heterodimer, the functionality of p66 is necessary while the functionality of the p51 subunit is not needed. Conversely, p51 is not able to catalyze DNA synthesis when associated with p66, and yet when the p66 protein is absent, p51 can function. These conclusions applied to DNA synthesis on heteropolymeric RNA and DNA templates.

Proteolytic release and crystallization of the RNase H domain of human immunodeficiency virus type 1 reverse transcriptase

The RNase H domain of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase was released from recombinant DHFR-RNase H fusion protein by the action of HIV-1 protease and crystallized as large trigonal prisms that diffract x-rays to at least 2.4-A resolution. The protease cleavage occurred 18 residues away from the Phe440-Tyr441 site reported to be processed during maturation of the reverse transcriptase heterodimer. Mutagenesis of the protease-sensitive region (residues 430-440), which is part of the crystallized domain, indicates that any alteration of the wild-type sequence results in increased proteolysis of the p66 subunit. A model of asymmetric processing in HIV-1 reserve transcriptase which involves partial unfolding of the RNase H domain is proposed based on these results and the recently reported three-dimensional structure of this domain.

Crystal structure of the ribonuclease H domain of HIV-1 reverse transcriptase

The crystal structure of the ribonuclease (RNase) H domain of HIV-1 reverse transcriptase (RT) has been determined at a resolution of 2.4 A and refined to a crystallographic R factor of 0.20. The protein folds into a five-stranded mixed beta sheet flanked by an asymmetric distribution of four alpha helices. Two divalent metal cations bind in the active site surrounded by a cluster of four conserved acidic amino acid residues. The overall structure is similar in most respects to the RNase H from Escherichia coli. Structural features characteristic of the retroviral protein suggest how it may interface with the DNA polymerase domain of p66 in the mature RT heterodimer. These features also offer insights into why the isolated RNase H domain is catalytically inactive but when combined in vitro with the isolated p51 domain of RT RNase H activity can be reconstituted. Surprisingly, the peptide bond cleaved by HIV-1 protease near the polymerase-RNase H junction of p66 is completely inaccessible to solvent in the structure reported here. This suggests that the homodimeric p66-p66 precursor of mature RT is asymmetric with one of the two RNase H domains at least partially unfolded.

Reconstitution in vitro of RNase H activity by using purified N-terminal and C-terminal domains of human immunodeficiency virus type 1 reverse transcriptase

Two constituent protein domains of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase were expressed separately and purified to homogeneity. The N-terminal domain (p51) behaves as a monomeric protein exhibiting salt-sensitive DNA polymerase activity. The C-terminal domain (p15) on its own has no detectable RNase H activity. However, the combination of both isolated p51 and p15 in vitro leads to reconstitution of RNase H activity on a defined substrate. These results demonstrate that domains of HIV-1 reverse transcriptase are functionally interdependent to a much higher degree than in the case of reverse transcriptase from Moloney murine leukemia virus.

Affinity chromatography with biotinylated RNAs

Small, reversibly biotinylated RNAs as described here are versatile ligands for affinity chromatography of RNA-binding components. These RNAs can be attached to a solid support by binding to avidin and used as ligands, or they may be hybridized to another RNA which acts as the ligand. The incorporation of a disulfide bond in the linker arm connecting biotin to the RNA makes it possible to dissociate the RNA from avidin under mild conditions. Our results regarding the binding and elution of the biotinylated RNA may be applied to other, reversibly biotinylated molecules.

High-level expression of self-processed HIV-1 protease in Escherichia coli using a synthetic gene

A synthetic gene coding for HIV-1 protease (PR) has been constructed and a system for its efficient expression in E. coli has been established: PR is synthesized as a fusion protein with E. coli dihydrofolate reductase under the control of a bacteriophage T7 promoter. The synthetic gene was constructed to enable rapid construction of defined mutants by restriction fragment replacement. A set of mutants has been constructed which may facilitate elucidation of the mechanism of PR self-cleavage from polyprotein precursors. We have demonstrated that the C-terminal residue (Phe99 in the native sequence) of the processing intermediate is absolutely required for subsequent cleavage at the N-terminal cleavage site. The potential structural role of this residue is discussed with reference to the recently published HIV-1 PR structure.