Construction and recovery of viable retroviral genomes carrying a bacterial suppressor transfer RNA gene.

The integration of retroviral genomes into cellular DNA can induce mutations by altering the expression of nearby cellular genes and can serve to identify the gene affected. The construction of a retrovirus that stably carries a suppressor transfer RNA gene from Escherichia coli has allowed facile recovery of the viral genome in vectors marked with amber mutations. This virus can be used for rapid isolation of cellular sequences at the site of proviral insertion.

Insertion mutagenesis of embryonal carcinoma cells by retroviruses.

Mutagenesis was studied in cultured F9 embryonal carcinoma cells infected with a variant of Moloney murine leukemia virus. Proviral insertion induced the inactivation of the hypoxanthine phosphoribosyltransferase locus, and the virus was used to isolate the mutated genes rapidly. Mutagenesis by these methods may be useful for the genetic dissection of the various mammalian cell phenotypes.

Mutants of murine leukemia viruses and retroviral replication.

The analysis of retroviral mutants has played a critical role in the development of our understanding of the complex viral life cycle. The most fundamental result of that analysis has been the definition of the replication functions encoded by the viruses. From a biochemical examination of a particular step in the life cycle it is difficult to determine, for example, whether that step is catalyzed by a viral or a host enzyme; but the isolation of a viral mutant defective in that step can firmly establish that a viral function is involved. In this way many facts about the viruses have been established. We know that reverse transcriptase is encoded by the virus; that RNAase H and DNA polymerase activities reside on the same gene product; that processing of many precursor proteins is mediated by a viral proteinase; and that establishment of the integrated provirus requires a viral protein. The list of functions mediated by viral enzymes has largely been defined by the mutants isolated and studied in various laboratories. The second significant result of the studies of viral mutants has been the assignation of the replication functions to particular viral genes, and then more specifically to particular domains of these genes. Mutants and viral variants have been essential in the determination, for example, that the gag protein is the critical gene product for the assembly of a virion particle; that the env protein is the determinant of species specificity of infection; or that the LTR is a major determinant of tissue tropism and leukemogenicity. The subdivisions of functions within a given gene have similarly hinged on mutants. Genetic mapping was needed to establish that P30 is the most important region for assembly; that the proteinase and integrase functions reside, respectively, in the 5' and 3' portions of the pol gene; and that the glycosylated gag protein is dispensable for replication. A third important area of knowledge has depended heavily on viral mutants: the determination of host functions and proteins that interact with viral proteins. Variant viruses with altered or restricted host ranges serve to define differences between pairs of different host cells, and the mapping of the viral mutations serves to define the viral protein important in that interaction with the host. These studies are only in their infancy, but it is clear that substantial efforts will be made to further analyze these host functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Filamentous phage displaying the extracellular domain of the hLH/CG receptor bind hCG specifically.

We have expressed the extracellular binding domain of the human LH/CG receptor as a fusion with the cpIII filamentous phage coat protein. These fusion phage are able to specifically bind ELISA plates coated with hCG. Preincubation of the phage with hCG before exposure to the coated plates inhibited binding of phage, whereas human FSH had no effect. Furthermore, addition of anti-hLH/CG receptor antisera inhibited binding of phage to the plates. We have also tested the effect of monoclonal antibodies to hCG on phage binding. Monoclonal antibodies that can bind hCG when it is bound to native receptor do not inhibit phage binding. These include B105 and A109 Alternatively, B107 and B109 have an inhibitory effect on phage binding. They cannot bind hCG when it is bound to receptor and are likely directed against epitopes at or near the receptor binding interface. Therefore, these fusion phage exhibit the same binding specificity as the wild-type receptor and likely display the extracellular domain of the hLH/CG receptor on their surface in the native conformation. Phage display is a potentially useful tool for studying the hLH/CG receptor structure and in screening for synthetic compounds that compete with hormone for receptor binding.

Development of a fluorescence based high throughput assay for antagonists of the human chorionic gonadotropin receptor extracellular domain: analysis of peptide inhibitors.

A simple method for prompt fluorescent detection of inhibitors of human chorionic gonadotropin (hCG) binding to the extracellular domain of the human luteinizing hormone/chorionic gonadotropin (hLH/CG) receptor was developed for high throughput screening (HTS). Construction and analysis of a recombinant phage that displays the extracellular binding domain of the hLH/CG receptor on its surface and specifically binds hCG was previously described. To facilitate the identification of molecules that disrupt the interaction of hCG with its receptor, a method for prompt fluorescent detection of these phage bound to hCG was developed. This technique is extremely sensitive and employs fluorescent labels (PBXL dyes) that are derived from red and blue-green algae. Antibodies labeled with PBXL dye were able to specifically detect phage that display the extracellular domain of the hLH/CG receptor when bound to hCG immobilized in 96-well microplates. Decreases in fluorescence correlate with the concentration of exogenous hCG or hCG antagonists in the assay. This prompt fluorescence detection assay was optimized in a 96-well format as a model system for HTS applications that target the receptors for the group of hormones known as the gonadotropins. Low-affinity molecules that disrupt binding of the phage-displayed receptor extracellular domain to hCG can be rapidly identified in this high throughput screen.

Reverse transcription of retroviral genomes: mutations in the terminal repeat sequences.

The process of reverse transcription of retroviral genomes begins with the synthesis of a short DNA molecule near the 5' end of the RNA template. This molecule, termed minus-strand strong-stop DNA, is then translocated to the 3' end of the viral RNA by means of a repeated sequence, the R region, present at both ends of the template. The translocation should result in the transfer of genetic information from the 5' R region to the 3' R region. We have generated a series of mutants of Moloney murine leukemia virus with alterations in the R regions by in vitro mutagenesis of a cloned DNA copy of the viral genome. The altered DNAs were introduced into mouse cells by transfection, and the translocation of the mutations during viral replication was assessed. Some mutations were not transferred from the 5' R region to the 3' R region; these results were not in accord with current models for reverse transcription. The results can be explained if DNA molecules shorter than strong-stop DNA, formed by premature termination of synthesis, are sometimes translocated. A number of mutants with large deletions in the R region were tested and were able to replicate with normal strong-stop DNA translocation. Thus, short stretches of homology can be used by the virus to carry out strong-stop translocations.

Construction of mutants of Moloney murine leukemia virus by suppressor-linker insertional mutagenesis: positions of viable insertion mutations.

A highly efficient method for the generation of insertion mutations is described. The procedure involves the use of a 220-base-pair (bp) EcoRI fragment bearing the SuIII+ suppressor tRNA gene as an insertional mutagen. The plasmid DNA to be mutagenized is linearized by a variety of means, and the suppressor fragment is ligated into the site of cleavage. Successful insertion mutants can be readily detected in Escherichia coli carrying lac- amber mutations on MacConkey lactose plates; virtually 100% of the red colonies contain insertions of the fragment. Subsequent removal of the SuIII+ gene and recyclization leaves a 12-bp insertion if the original cleavage was blunt-ended and a 9-bp insertion if the original cleavage generated 3-bp cohesive termini. This technique, as well as conventional linker mutagenesis with decamer and dodecamer linkers, was used to generate a large library of insertion mutations in cloned DNA copies of the genome of Moloney murine leukemia virus. A number of viable mutants were isolated bearing 9-, 10-, and 12-bp insertions in various domains of the genome. The map positions of the viable mutations suggest that the viral long terminal repeats and portions of the gag and env genes are quite insensitive to alteration. Although most of the mutations were stable for many passages, some of the mutants lost the inserted DNA; we presume that the insertion was somewhat deleterious in these mutants and that continued passage of the virus selected for overgrowth by a revertant.

The palindromic LTR-LTR junction of Moloney murine leukemia virus is not an efficient substrate for proviral integration.

We generated viral constructs to test the hypothesis that the major substrate on retroviral DNA that is utilized for proviral DNA integration is the palindromic sequence, termed the LTR-LTR junction, normally present in circular molecules formed by joining the two termini of linear proviral DNA. Recombinant viral genomes were built which carried a selectable marker and an extra copy of the LTR-LTR junction from a cloned circular provirus. The junction sequence in each case was positioned such that its use during integration would lead to an easily detected, aberrantly integrated proviral DNA. Analysis of DNA from cells infected with the virus constructs showed that the introduced junction sequence is used at least 1,000-fold less efficiently than the natural sequences at the ends of the genome. This suggests that a linear or more exotic DNA intermediate is most likely the true precursor for the integration reaction.

A temperature-sensitive mutation constructed by "linker insertion" mutagenesis.

The in vitro mutagenesis of cloned DNAs allows the formation of virtually any specific mutation, but no method has been found which might routinely lead to the important phenotype of temperature sensitivity. We have studied three linker insertion mutations in the envelope gene of Moloney murine leukemia virus (M-MuLV), and found that one was exquisitely temperature-sensitive for plaque formation. We suggest that the construction of short insertion mutations may be a fruitful approach for the generation of temperature-sensitive phenotypes in cloned genes.

High-level bacterial expression of a natively folded, soluble extracellular domain fusion protein of the human luteinizing hormone/chorionic gonadotropin receptor in the cytoplasm of Escherichia coli.

We have expressed the extracellular domain of the human luteinizing hormone/chorionic gonadotropin (hLH/CG) receptor as a fusion protein with thioredoxin in the cytoplasm of an Escherichia coli strain that contains mutations in both the thioredoxin reductase and glutathione reductase genes. The chimeric protein isolated following induction of expression is purified in a soluble form and binds hCG with an affinity approximating that of native receptor. This truncated form of the receptor displays the same specificity as intact hLH/CG receptor and does not bind human follicle stimulating hormone. This cytoplasmically produced protein is expressed at levels that exceed 10 mg/L. Expression of properly folded extracellular domain of the hLH/CG receptor in the cytoplasm of E. coli allows the facile and economic purification of large quantities of material. This will facilitate the determination of the structure of the hormone-binding domain of the glycoprotein receptor as well as the production of epitope-specific antibodies.