Cellular fitness is determined by ribosomal protein S12-mediated release of a truncated Xrp1

Multicellular tissues require continuous optimization to maintain their structure and function by actively eliminating viable, yet unfit cells via a mechanism known as cell competition. During this process, unfit ‘loser’ cells commonly upregulate the C/EBP family transcription factor Xrp1, which causes their elimination, indicating that intracellular Xrp1 levels determine cellular fitness. However, despite its central role, the mechanism by which Xrp1 is upregulated in unfit cells remains unknown. Here, we show that Xrp1 is upregulated through a previously unrecognized post-transcriptional regulatory mechanism mediated by the ribosomal protein S12 (RpS12). Surprisingly, Xrp1 mRNA is abundantly expressed even in wild-type cells, yet it is not translated. This is due to an upstream open reading frame (uORF) located in the 5’UTR of the initial coding exon of Xrp1 mRNA, which inhibits the translation of the main Xrp1 ORF. Intriguingly, in unfit cells, RpS12 causes splicing-mediated skipping of this initial exon, leading to the use of an alternate start codon that generates a short isoform of Xrp1 protein, causing cell death. Notably, protein structural analysis reveals that RpS12 is highly homologous to that of a spliceosomal component SNU13, suggesting the role of RpS12 in directly regulating alternative splicing of Xrp1 mRNA. Our findings thus provide not only a mechanistic insights into how cellular fitness is determined but also how tissue optimization is achieved.