Host cell RNA packaging in retroviruses
Retroviruses, including Moloney murine leukemia virus (M-MuLV) and human immunodeficiency virus type 1 (HIV-1), have RNA as their genome. Two complete copies of the RNA genome are specifically packaged. These make up the bulk of the RNA within virions. In addition, up to 30% of the RNAs in retroviral particles are host RNAs. Previous studies of the RNA content of several retroviruses shed some light on the species of RNAs present in virions. However, the complete profile of host RNA in virions is unknown and it was unclear how abundant each host RNA was within viral particles. Except for the tRNA primer (which initiates reverse transciption) the mechanisms of host RNA packaging and the possible functions of host RNAs within retroviruses remained unclear.
The aim of this thesis was to explore host RNA packaging in retroviruses. Using M-MuLV and HIV-1 as the primary models, small RNAs in virions were identified and quantified. Also, mechanisms of host RNA selection, including viral and host cell determinants were investigated. The overall implications of the findings as they relate to the retroviral replication and pathogenesis were discussed.
Experiments described in this thesis demonstrate that host RNAs packaged in retroviruses are incorporated non-randomly, with some RNAs being enriched while others are underrepresented. Based on a comparison of their ratios in cells and virions, the following RNAs are enriched: the signal recognition particle RNA (7SL), 5S ribosomal RNA, U6 spliceosomal RNA, B1 RNA, tRNA(Pro) and tRNA(Gln). U1, 02, U4 and U5 spliceosomal RNAs, tRNAs (Met) and (Val), as well as 5.8S and 18S ribosomal RNAs are underrepresented in virus particles. An examination of possible viral factors involved in RNA packaging reveals that the determinants governing viral RNA packaging are dispensable for host RNA packaging. Furthermore, host RNAs in retroviruses, although they exist in cells as part of ribonucleoprotein (RNP) complexes, do not appear to package their component RNP proteins. Altogether these data suggest a role for host RNAs during retroviral replication.