Functional dissection of adenovirus VAI RNA

During the course of adenovirus infection, the VAI RNA protects the translation apparatus of host cells by preventing the activation of host double-stranded RNA-activated protein kinase, which phosphorylates and thereby inactivates the protein synthesis initiation factor eIF-2. In the absence of VAI RNA, protein synthesis is drastically inhibited at late times in infected cells. The experimentally derived secondary structure of VAI RNA consists of two extended base-paired regions, stems I and III, which are joined by a short base-paired region, stem II, at the center. Stems I and II are joined by a small loop, A, and stem III contains a hairpin loop, B. At the center of the molecule and at the 3' side, stems II and III are connected by a short stem-loop (stem IV and hairpin loop C). A fourth, minor loop, D, exists between stems II and IV. To determine sequences and domains critical for function within this VAI RNA structure, we have constructed adenovirus mutants with linker-scan substitution mutations in defined regions of the molecule. Cells infected with these mutants were analyzed for polypeptide synthesis, virus yield, and eIF-2 alpha kinase activity. Our results showed that disruption of base-paired regions in the distal parts of the longest stems, I and III, did not affect function, whereas mutations causing structural perturbations in the central part of the molecule containing stem II, the proximal part of stem III, and the central short stem-loop led to loss of function. Surprisingly, one substitution mutant, sub742, although dramatically perturbing the integrity of the structure of this central portion, showed a wild-type phenotype, suggesting that an RNA with an alternate secondary structure is functional. On the basis of sensitivity to single-strand-specific RNases, we can derive a novel secondary structure for the mutant RNA in which a portion of the sequences may fold to form a structure that resembles the central part of the wild-type molecule, which suggests that only the short stem-loop located in the center of the molecule and the adjoining base-paired regions may define the functional domain. These results also imply that only a portion of the VAI RNA structure may be recognized by the host factor(s).