The genome of the human immunodeficiency virus (HIV) contains several open reading frames (ORFs) not present in other viruses. The 'A' gene1, also known as Q2 P'3, ORF-14 or sor 5, partially overlaps the pol gene; its protein product has a relative molecular mass of 23,000 (Mr 23K) and is present in productively infected cells7-10. The function of this protein is unclear; mutant viruses deleted in 'A' replicate in and kill CD4+ lymphocyte lines8, but the high degree of conservation of the deduced amino-acid sequence in nine different HIV isolates (80%) and the presence of analogous genes in HIV-211 and other lentiviruses suggest that the gene function is an important one. Here we describe a mutant virus deficient in the 'A' gene which produces virion particles normally; however, the particles are & sm;1,000 times less infective than wild type. Transcomplementation experiments partially restore infectivity. The mutant virus spreads efficiently when virus-producing cells are co-cultivated with CD4+ lymphocytes, however, indicating that HIV can spread from cell to cell in a mechanism that does not require the 'A' gene product and probably does not require the production of infective virus particles.
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