Extensively overlapping reading frames in a second mammalian gene

Given the roominess of the mammalian genome, the discovery of a gene that saves space by being translated in two reading frames over a distance of 356 codons is unexpected. Klemke et al . (2001) have found that the rat XLαs gene, previously shown to encode a G protein α subunit, encodes a second protein—dubbed ALEX—in the +1 reading frame. The start codon for ALEX (AUGALEX) is 32 nt downstream from AUGXLαs (Figure 1A). Even among viruses, where overlapping genes are employed to compress the genome to a packageable size, such a long dual‐coding domain is rare. The 105 codon overlap in the INK4A/ARF locus is the only other extensive dual‐coding sequence identified thus far in mammals (Sharpless and DePinho, 1999). Whereas the two proteins produced from the INK4A/ARF locus are translated from separate mRNAs (Figure 1B), only one mRNA has been found for the XLαs/ALEX locus.Figure 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 …