Eukaryotic translation can be initiated either by a cap-dependent mechanism or by internal ribosome entry, a process by which ribosomes are directly recruited to structured regions of mRNA upstream of the initiation codon. We analysed the 5' untranslated region (UTR) of the proto-oncogene N-myc, and demonstrated by transfections in a dicistronic vector system that it contains a potent internal ribosome entry segment (IRES). The IRES is similar in length to the c-myc IRES and the activities of these IRESs are comparable in non-neuronal cells. Transfections were also carried out in cell lines derived from neuroblastomas, in which N-myc is expressed, and in a neuronal precursor cell line. In these cells the N-myc IRES is up to seven times more active than that of c-myc, suggesting that neuronal-specific non-canonical trans-acting factors are used by the N-myc but not the c-myc IRES. N-myc expression is increased by gene amplification in many neuroblastomas, but this is the first example of a translational mechanism by which N-myc expression could be further increased. The discovery of an IRES that displays enhanced activity in neuronal cell lines has important potential as a tool for protein expression in neural tissue.
