The Dmp8-Dmp18 bicistron messenger RNA enables unusual translation during cellular stress

Alternatives to the cap mechanism in translation are often used by viruses and cells to allow them to synthesize proteins in events of stress and viral infection. In Drosophila there are hundreds of polycistronic messenger RNA (mRNA), and various mechanisms are known to achieve this. However, proteins in a same mRNA often work in the same cellular mechanism, this is not the case for Drosophila's Swc6/p18^Hamlet homolog Dmp18, part of the SWR1 chromatin remodeling complex, who is encoded in a bicistronic mRNA next to Dmp8 (Dmp8-Dmp18 transcript), a structural component of transcription factor TFIIH. The organization of these two genes as a bicistron is conserved in all arthropods, however the length of the intercistronic sequence varies from more than 90 to 2 bases, suggesting an unusual translation mechanism for the second open reading frame. We found that even though translation of Dmp18 occurs independently from that of Dmp8, it is necessary for Dmp18 to be in that conformation to allow its correct translation during cellular stress caused by damage via heat-shock and UV radiation.