Uncapped 5′ ends of mRNAs targeted by cytoplasmic capping map to the vicinity of downstream CAGE tags

In mammalian transcriptomes approximately 25% of 5′ ends determined by Capped Analysis of Gene Expression (CAGE) map to locations within spliced exons. The current study sought to determine if the cytoplasmic capping complex participates in generating these downstream CAGE tags. 5′-RACE was used to amplify the uncapped ends of target transcripts that accumulate when cytoplasmic capping is blocked. Sequencing of these RACE products mapped the positions of uncapped ends either exactly to or just downstream of archived CAGE tags. These findings support a role for cytoplasmic capping in generating the downstream capped ends identified by CAGE.