TRANSCRIPTION TERMINATION AND POLYADENYLATION IN RETROVIRUSES

The provirus structure of retroviruses is bracketed by long terminal r epeats (LTRs). The two LTRs (5' and 3') are identical in nucleotide se quence and organization. They contain signals for transcription initia tion as well as termination and cleavage/polyadenylation. As in eukary otic pre-mRNAs, the two common signals, the polyadenylation signal, AA UAAA, or a variant AGUAAA, and the G + U-rich sequence are present in all retroviruses- However, the AAUAAA sequence is present in the U3 re gion in some retroviruses and in the R region in other retroviruses. A s in animal cell RNAs, both AAUAAA and G + U-rich sequences apparently contribute to the 3'-end processing of retroviral RNAs. In addition, at least in a few cases examined, the sequences in the U3 region deter mine the efficiency of 3'-end processing. In retroviruses in which the AAUAAA is localized in the R region, the poly(A) signal in the 3' LTR but not the 5' LTR must be selectively used for the production of gen omic RNA. It appears that the short distance between the 5' cap site a nd polyadenylation signal in the 5' LTR precludes premature terminatio n and polyadenylation. Since 5' and 3' LTRs are identical in sequence and structural organization yet function differently, it is speculated that flanking cellular DNA sequences, chromatin structure, and bindin g of transcription factors may be involved in the functional divergenc e of 5' and 3' LTRs of retroviruses.