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Previous studies have shown that certain HIV-1 Gag mutants can interfere with the production of infectious HIV-1 when coexpressed with wild-type virus. In this paper, we studied two mutants of HIV-1 for their ability to interfere with the production of wild-type virus. Both mutants lack the entire matrix domain of gag and either lack [myr(-)MA(-)] or contain [myr(+)MA(-)] an amino-terminal myristate (myr) addition sequence at the beginning of the capsid domain. Previously we have demonstrated that expression of both mutant constructs leads to assembly and release of mutant viruses, although only myr(+)MA(-) particles are released efficiently. Particles produced by both matrix-deficient mutants are noninfectious and poorly incorporate and/or retain viral envelope glycoproteins. In this study, we further show that expression of myr(+)MA(-), but not myr(-)MA(-) interferes with wild-type HIV-1 virus production in transient expression assays. Our data suggest that wild-type and myristylated MA(-) Gag protein interacts at some point during assembly and that Gag myristylation has a greater effect on the assembly pathway than the matrix domain.

The human foamy or spumaretrovirus (HFV) is a complex retrovirus that codes for the three retroviral genes gag, pol, and env and the regulatory and accessory bel genes. A particular feature of HFV gene expression was recently described: not only does the HFV provirus contain the classical retroviral long terminal repeat promoter, a second functionally active promoter is present in the env gene upstream of the bel genes (M. Löchelt, W. Muranyi, and R. M. Flügel, 1993, Proc. Natl. Acad. Sci. USA 90, 7317-7321). Both, the HFV long terminal repeat promoter I and internal promoter II depend upon the HFV transcriptional transactivator Bel 1 for efficient gene expression. The internal promoter directs the synthesis of functionally active Bel 1 transactivator and Bet proteins that are expressed early after HFV infection. In this report, it is shown that mutation of the promoter II TATA box resulted in HFV proviral clones with a reduction in infectivity by a factor of approximately 100. Gene expression by promoter II TATA box mutant HFV proviruses was reduced. HFV proviruses with the mutated promoter II TATA box used cryptic start sites of transcription upstream of the original promoter II TATA box, resulting in an inefficient and less accurate transcriptional initiation. The reduced HFV structural gene expression by the mutated HFV proviruses was relieved by providing Bel 1 protein in trans. This demonstrates that HFV promoter II-directed Bel 1 expression is important for producing the high levels of Bel 1 that increases virus replication.