We have constructed a series of deletion mutations in the p30 and plO 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 gagpol 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. MATERIALS AND METHODS Cells and virus. NIH 3T3 cells, M23 cells (26), and XC (21) fibroblasts were grown in Dulbecco modified Eagle medium (GIBCO Laboratories) with 10% calf serumn (M. A. Bioproducts) and 50 ,ug of gentamicin per ml. M-MuLV released by NIH 3T3 clone 4 (25) was the source of wild-type virus. Cloned DNAs. Plasmid p8.2 was the wild-type clone of M-MuLV used in these studies (25, 28). Plasmid pSV2neo (gift of P. Berg) contains the kanamycin resistance gene of Tn3 linked to a simian virus 40 promoter (30).
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