the 5' end (U5) or the 3' end (U3). Reverse transcription of the viral RNA is initiated by a tRNA primer which is hydrogen-bonded to a region of the genome known as the primer binding site (PBS), localized between the end of the 5' LTR and the beginning of gag. Reverse transcription then proceeds towards the 5' end of this LTR and then jumps to the 3' end of the same or another viral genome. The resulting linear DNA copy is flanked by two LTRs and is then integrated into the host genome.

The core structural proteins of replication-competent retroviruses are encoded by the gag gene, the viral enzymes reverse transcriptase, proteinase and integrase are encoded by the pol gene, and the two envelope proteins by the env gene (Figure 1). The gag and pol gene products are translated from a single full-length viral RNA as a precursor polyprotein which is then cleaved by the retroviral proteinase to yield the individual proteins. In the case of murine leukemia virus (MLV), this common precursor is produced by occasionally suppressing the stop codon at the end of gag. For the other retroviruses 'riboso-mal frameshifting' is required to change the reading frame between gag and pol. In the case of human T lymphotropic virus type I (HTLV-I), a similar frame-shifting event occurs between the proteinase gene at the 5' end of pol and the reverse transcriptase gene.

Typically, gag encodes three proteins, although in some cases there are further processing products. The first of these, the matrix protein, is encoded at the N-terminus of the gag precursor protein and serves to anchor the core complex underneath the cell membrane, often by a myristylated N-terminal glycine residue. The second protein encoded by gag, the capsid protein, is the major structural core protein, whereas the third, C-terminal gag protein interacts with the genomic RNA and serves to dimerize the genomic RNA copies and to package them selectively in the viral particle. In the dimerized state the genomic RNA cannot serve as a template for translation.

For C-type viruses and lentiviruses, the cleavage of the gag-pol precursor, performed by the viral proteinase, takes place mostly after budding from the cell membrane. This cleavage is accompanied by the 'maturation' of viral particles, as visualized by electron microscopy. In contrast, B-type and D-type

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