Fig. 1. Dual-coding mRNAs from eukaryotic cells and viruses. Possible mechanisms for translating the cellular mRNAs (A, B) are discussed in the text. In the mouse XLαs/ALEX transcript, translation of XLαs might start upstream from the indicated point; see DDBJ/EMBL/GenBank entry AJ245739. With viral mRNAs (C), leaky scanning allows access to the downstream start codon not only when the first AUG resides in a poor context (TYMV; Weiland and Dreher, 1989) but also, to a limited extent, when the first AUG resides in a good but not perfect context, as in BCV. In the latter case, the low-level translation of I-protein was suppressed when the N-protein start site was changed from aggAUGu to the more optimal aggAUGg (Senanayake and Brian, 1997). Coding (open boxes) and noncoding domains (filled boxes) are not drawn to scale.