Absence of internal ribosome entry site-mediated tissue specificity in thetranslation of a bicistronic transgene

The 5' noncoding regions of the genomes of picornaviruses form a complex st ructure that directs cap-independent initiation of translation. This struct ure has been termed the internal ribosome entry site (IRES), The efficiency of translation initiation was shown, ire vitro, to be influenced by the bi nding of cellular factors to the IRES, Hence, we hypothesized that the IRES might control picornavirus tropism. In order to test this possibility, we made a bicistronic construct ire which translation of the luciferase gene i s controlled by the IRES of Theiler's murine encephalomyelitis virus. In vi tro, we observed that the IRES functions in various cell types and in macro phages, irrespective of their activation state. In vivo, we observed that t he IRES is functional in different tissues of transgenic mice. Thus, it see ms that the IRES is not an essential determinant of Theiler's virus tropism , On the other hand, the age of the mouse could be critical for IRES functi on. Indeed, the IRES was found to be more efficient in young mice. Picornav irus IRESs are becoming popular tools in transgenesis technology, since the y allow the expression of two genes from the same transcription unit. Our r esults show that the Theiler's virus IRES is functional in cells of differe nt origins and that it is thus a broad-spectrum tool. The possible age depe ndency of the IRES function, however, could be a drawback for gene expressi on in adult mice.