Alfalfa mosaic virus temperature-sensitive mutants. II. Early functions encoded by RNAs 1 and 2

Expression of alfalfa mosaic virus cDNA1 and 2 in transgenic tobacco plants

Chimeric genes composed of DNA complementary to alfalfa mosaic virus (AIMV) RNAs 1 or 2, the CaMV 35 S promoter, and the nos polyadenylation signal were transferred to the genome of Nicotiana tabacum cv. Samsun NN by means of the Agrobacterium tumefaciens transformation system. Transformants contained intact copies of the viral genes and accumulated transcripts of approximately the size predicted from the cloning procedure. Using antisera raised against synthetic peptides corresponding to the C-terminal parts of AIMV P1 and P2, it was not possible to detect viral translation products in the transformants. However, transgenic protoplasts containing cDNA1 were able to complement an infection by the AIMV nucleoproteins containing RNAs 2 and 3, indicating that biologically active P1 accumulates in these protoplasts. Upon inoculation with AIMV strains 425 or YSMV, the cDNA1- and cDNA2-transformed plants became infected to a level similar to that of nontransformed or vector-transformed control plants.

Regulation of protein synthesis in virus-infected animal cells

This chapter summarizes the structural features that govern the translation of viral mRNAs: where the synthesis of a protein starts and ends, how many proteins can be produced from one mRNA, and how efficiently. It focuses on the interplay between viral and cellular mRNAs and the translational machinery. That interplay, together with the intrinsic structure of viral mRNAs, determines the patterns of translation in infected cells. It also points out some possibilities for translational regulation that can only be glimpsed at present, but are likely to come into focus in the future. The mechanism of selecting the initiation site for protein synthesis appears to follow a single formula. The translational machinery displays a certain flexibility that is exploited more frequently by viral than by cellular mRNAs. Although some of the parameters that determine efficiency have been identified, how efficiently a given mRNA will be translated cannot be predicted by summing the known parameters.