Multi-functional RNA binding proteins regulate and coordinate the many steps of RNA metabolism. Accurate functioning of these processes is vital in cells and misregulation has been linked to many human diseases. RNA binding proteins contain multiple RNA binding domains. The ability to perform multiple functions depends on the recognition of a diverse range of targets and domains are used combinatorially to achieve this. In this thesis I ask how the sequence specificity of low affinity RNA-binding domains and the interplay between said domains plays a role in RNA target selectivity. Within this question I focus on three proteins; TUT4, a uridyl transferase involved in the regulation of both non-coding RNAs and histone mRNA; FMRP, a translational repressor whose loss in cells is the cause of Fragile X Syndrome; and RBM10 a regulator of alternative splicing and miRNA biogenesis. I found that through the use of separate RNA binding domains both TUT4 and RBM10 are able to exert flexibility in target recognition; TUT4 by using two CCHC-type zinc fingers, working independently to recognise short RNA stretches; and RBM10 by using different subsets of domains to recognise either specific high affinity splice site sequences or pre-miRNAs. In FMRP the determination of the sequence specificity of KH1 allowed us to isolate its contribution to target selection. In a secondary objective, looking at methodologies used in RNA-protein interaction, SIA was improved to make it both less laborious and to reduce the sample requirements, and with FMRP a novel mutational strategy was used in combination with SIA to determine the sequence specificity of this low affinity domain. In summary these data extend our understanding of the RNA binding mechanisms of the three systems studied and introduces improved or novel methodologies to the future study of protein-RNA interactions
