Four novel U RNAs are encoded by a herpesvirus

Marmoset T lymphocytes transformed by herpesvirus saimiri contain the first virally encoded U RNAs (called HSURs) to be identified. HSURs assemble into small nuclear ribonucleoproteins of low abundance (≤ 2 × 10⁴ copies/cell). They bind proteins with Sm determinants and acquire a 5′ trimethylguanosine cap structure. The sequences of HSUR 1 (143 nucleotides), HSUR 2 (115 nucleotides), HSUR 3 (76 nucleotides), and HSUR 4 (106 nucleotides) are related to each other but are distinct from any previously characterized cellular U RNA. The viral genes encoding the HSURs possess conserved enhancer, promoter, and 3′ end formation signals unique to U RNA genes. HSUR 1 and HSUR 2 have a similar 5′ end sequence that exhibits perfect complementarity to the highly conserved AAUAAA polyadenylation signal. Oligonucleotide directed RNAase H degradation indicates that this 5′ end region is available for base pairing interactions within the HSUR 1 and HSUR 2 snRNP particles.