Mutations in gag proteins P12 and P15 of Moloney murine leukemia virus block early stages of infection

A deletion mutation in the 5' part of the pol gene of Moloney murine leukemia virus blocks proteolytic processing of the gag and pol polyproteins

Deletion mutations in the 5' part of the pol gene of Moloney murine leukemia virus were generated by restriction enzyme site-directed mutagenesis of cloned proviral DNA. DNA sequence analysis indicated that one such deletion was localized entirely within the 5' part of the pol gene, did not affect the region encoding reverse transcriptase, and preserved the translational reading frame downstream of the mutation. The major viral precursor polyproteins (Pr65gag, Pr200gag-pol, and gPr80env) were synthesized at wild-type levels in cell lines carrying the mutant genome. These cell lines assembled and released wild-type levels of virion particles into the medium. Cleavage of both Pr65gag and Pr200gag-pol precursors to the mature proteins was completely blocked in the mutant virions. Surprisingly, these virions contained high levels of active reverse transcriptase; examination of the endogenous reverse transcription products synthesized by the mutant virions revealed normal amounts of minus-strand strong-stop DNA, indicating that the RNA genome was packaged and that reverse transcription in detergent-permeabilized virions was not significantly impaired. Processing of gPr80env to gP70env and P15E was not affected by the mutation, but cleavage of P15E to P12E was not observed. The mutant particles were poorly infectious; analysis indicated that infection was blocked at an early stage. The data are consistent with the idea that the 5' part of the pol gene encodes a protease directly responsible for processing Pr65gag, and possibly Pr200gag-pol, to the structural virion proteins. It appears that cleavage of the gag gene product is not required for budding and release of virions and that complete processing of the pol gene product to the mature form of reverse transcriptase is not required for its functional activation.

Novel immunoglobulin heavy chains are produced from DJH gene segment rearrangements in lymphoid cells

It has been found that most immunoglobulin heavy(H)-chain gene diversity (D) segments carry their own 5' transcriptional promoter element. Transcription of rearrangements which use heavy-chain D and joining (J) segments in B-lymphoid cells ultimately leads to the production of a Dmu messenger RNA which contains the DJH segments linked to the mu heavy-chain constant region. If the D is joined to the JH segment in the appropriate translational reading frame, this Dmu mRNA is translated to yield a short Dmu protein with a variable DJH N-terminus.

Construction and analysis of deletion mutations in the pol gene of Moloney murine leukemia virus: a new viral function required for productive infection

We have used in vitro mutagenesis to explore the functions of the gene products encoded by the pol gene of Moloney murine leukemia virus (M-MuLV). Deletions were constructed at a variety of positions in the gene, and the altered DNA copies of the viral genome were introduced into mouse cells by cotransformation. The mutants could be divided into two classes depending on the phenotype and map position of the deletion within the pol gene. Mutants with deletions mapping in the 5' portion of the gene were found to be completely deficient in reverse transcriptase activity. Mutants mapping in the 3' portion of the gene, however, assembled and released virions with normal levels of reverse transcriptase and RNAase H activities. When applied to permissive cells, these virions directed the synthesis of all three forms of unintegrated viral DNA: full-length, double-stranded linear DNA and the two circular forms with one and two copies of the long terminal repeat sequences. The infection was arrested at this point and the infected cells did not become producers of virus. Thus the 3' portion of the pol gene encodes a polypeptide with a function distinct from that of reverse transcriptase, which is not required for synthesis of viral DNA but is essential for establishment of that DNA in a stable, active form in the infected cell. We suggest that this function may be the integration of the proviral DNA.

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.

Mutations in the gag gene of Moloney murine leukemia virus: effects on production of virions and reverse transcriptase

We have constructed a series of deletion mutations in the p30 and p10 domains of the gag gene of Moloney murine leukemia virus. Mutants with deletions in P30 were completely defective in virion particle production even though an altered gag precursor protein is synthesized. This domain is apparently critical for particle formation. A mutant in P10 was able to release virion particles into the medium, and low levels of reverse transcriptase activity could be detected in these virions. To explore the effects of these mutations on the utilization of the gag-pol precursor, we have introduced these mutants into cells already releasing defective particles from an endogenous provirus which directs the synthesis of gag gene products and not pol gene products. The P10 mutant was capable of providing pol function as judged by the incorporation of high levels of reverse transcriptase into the particles and complete complementation for XC plaque formation. In contrast, the mutants in P30 were negative in this complementation test. Thus, those gag mutants which were unable on their own to assemble virion particles were also unable to contribute the gag-pol precursor to these particles. These mutations are the first to be mapped to the gag region which affect pol function, suggesting that the gag-pol precursor must be assembled before pol is functionally separated from the gag domain. The concordance of the effects of different mutations on both particle formation and gag-pol utilization suggests that similar domains of gag (namely, domains in the P30 region) are needed for these two processes.