Control of gene expression in space and time plays an important role in enabling cells to know where they are in the developing embryo and what to become, a process often referred to as cellular specification. Decades of research have demonstrated numerous layers of regulation in control of gene expression, at both the transcriptional and post-transcriptional level, which coordinate this process. Translational contro of gene expression has, on the contrary, received less experimental attention. Most notably, the prevailing dogma is that at the level of protein production, the ribosome -although an immensely complex molecular machine- possesses a constitutive rather than regulatory function in translating mRNAs. Our findings unexpectedly reveal that fundamental aspects of embryonic development and tissue patterning are instead controlled by a highly regulatory function of the ribosome. Importantly, we have shown that specialized ribosomes harboring a unique protein composition or activity confer tremendous specificity to how the mammalian genome is decoded into proteins. Our recent studies have also begun to elucidate how expression information encoded within the mRNA template confers gene regulatory potential by the ribosome to guide embryonic development. In particular, our research has identified novel RNA regulons embedded within the 5'UTRs of key developmental regulators, such as entire subsets of Homeobox (Hox) genes, which direct how gene products are translated in time and space to pattern the mammalian body plan. These findings transform our understanding of gene regulation and open a new portal of understanding into an additional layer of regulation embedded within vertebrate 5'UTRs vital to control of cell specification, tissue patterning, and embryonic development. In this proposal we will undertake a highly multidisciplinary approach to characterize this novel regulatory code for translational control of key developmental transcripts that primes them for post- transcriptional regulation. In Aim1 we will characterize a novel post-transcriptional circuitry required for control of Hox gene expression in time and space. In particular, we will undertake a multi-faceted genetic approach to uncover the functional roles of novel 5'UTR RNA Regulons including IRES-like and TIE elements in multiple Hox genes 5'UTRs towards key aspects of their expression in vivo and patterning of the vertebrate embryo. In Aim2 we will characterize the TIE element, a newly identified RNA regulatory element with remarkable potential to functionally specialize the translation of the mammalian genome. In Aim3 we will more broadly define the impact of RNA binding proteins on IRES-mediated translational control of key vertebrate developmental regulators. Together, these studies will open a new portal of understanding into the grammatical rules facilitated by unique RNA elements embedded within vertebrate 5'UTRs, which serves to expand developmental gene expression programs guiding organismal development.