ARS2 is a regulator of RNA polymerase II transcript processing through its role in the maturation of distinct nuclear cap-binding complex (CBC)-controlled RNA families. In this study, we examined ARS2 domain function in transcript processing. Structural modeling based on the plant ARS2 orthologue, SERRATE, revealed 2 previously uncharacterized domains in mammalian ARS2: an N-terminal domain of unknown function (DUF3546), which is also present in SERRATE, and an RNA recognition motif (RRM) that is present in metazoan ARS2 but not in plants. Both the DUF3546 and zinc finger domain (ZnF) were required for association with microRNA and replication-dependent histone mRNA. Mutations in the ZnF disrupted interaction with FLASH, a key component in histone pre-mRNA processing. Mutations targeting the Mid domain implicated it in DROSHA interaction and microRNA biogenesis. The unstructured C terminus was required for interaction with the CBC protein CBP20, while the RRM was required for cell cycle progression and for binding to FLASH. Together, our results support a bridging model in which ARS2 plays a central role in RNA recognition and processing through multiple protein and RNA interactions. The generation of mature RNA in the nucleus is a highly coordinated process that requires distinct complexes for the biogenesis of different RNA families. For RNA polymerase II (RNAP II) transcripts, a critical step is the addition of a 7-methylguanosine (m7G) cap to the nascent transcript (1), which is subsequently bound by CBP20 and CBP80 of the nuclear cap-binding complex (CBC) (2). CBC-controlled transcripts include mRNA, microRNA (miRNA), replication-dependent histone (RDH) mRNA, small nucleolar RNA (snoRNA), and small nuclear RNA (snRNA), each with its own unique processing requirements. Binding of the CBC to the m7G cap of these RNAs occurs cotranscriptionally, protects the transcripts from degradation, and plays a central role in recruiting the appropriate machinery for processing different RNA families (3-8). However, exactly how distinct RNA families are differentially recognized to allow for correct processing complex formation is not fully understood. Recently, a protein called ARS2 (or SRRT in humans) has been shown to be part of the CBC, and it plays an important role in the maturation of several distinct RNA families (8, 9).Clues to how ARS2 participates in recruiting different RNA processing machineries are beginning to emerge from biochemical and structural studies. ARS2 interacts directly with the assembled CBP20/80 cap complex to form a tertiary complex termed CBCA (9). One hypothesis is that ARS2 bridges the CBCA to the appropriate processing machinery by interacting with both protein and RNA elements. This hypothesis is based on studies of the ARS2 plant orthologue SERRATE in miRNA biogenesis. Both the Arg/Pro-rich N terminus and zinc finger (ZnF) domain of SERRATE are required for primary miRNA (primiRNA) binding (10, 11). Additionally, SERRATE, through its N terminus and ZnF, has been shown to bind directly to...