Trimethylguanosine capping selectively promotes expression of Rev-dependent HIV-1 RNAs.

5'-mRNA capping is an early modification that affects pre-mRNA synthesis/splicing, RNA cytoplasmic transport, and mRNA translation and turnover. In eukaryotes, a 7-methylguanosine (m7G) cap is added to newly transcribed RNA polymerase II (RNAP II) transcripts. A subset of RNAP II-transcribed cellular RNAs, including small nuclear RNA (snRNA), small nucleolar RNA (snoRNA), and telomerase RNA, is further hypermethylated at the exocyclic N2 of the guanosine to create a trimethylguanosine (TMG)-capped RNA. Some of these TMG-capped RNAs are transported within the nucleus and from the nucleus to the cytoplasm by the CRM-1 (required for chromosome region maintenance) protein. CRM-1 is also used to export Rev/RRE-dependent unspliced/ partially spliced HIV-1 RNAs. Here we report that like snRNAs and snoRNAs, some Rev/RRE-dependent HIV-1 RNAs are TMG-capped. The methyltransferase responsible for TMG modification of HIV-1 RNAs is the human PIMT (peroxisome proliferator-activated receptor-interacting protein with methyltransferase) protein. TMG capping of unspliced/partially spliced HIV-1 RNAs represents a new regulatory mechanism for selective expression.