Small nuclear RNAs (snRNAs) play essential roles in spliceosome assembly and splicing. Most snRNAs are transcribed by the DNAdependent RNA polymerase II (Pol II) and require 39-end endonucleolytic cleavage. We have previously shown that the Arabidopsis (Arabidopsis thaliana) Defective in snRNA Processing 1 (DSP1) complex, composed of at least five subunits, is responsible for snRNA 39 maturation and is essential for plant development. Yet it remains unclear how DSP1 complex subunits act together to process snRNAs. Here, we show that DSP4, a member of the metallo-b-lactamase family, physically interacts with DSP1 through its b-Casp domain. Null dsp4-1 mutants have pleiotropic developmental defects, including impaired pollen development and reduced pre-snRNA transcription and 39 maturation, resembling the phenotype of the dsp1-1 mutant. Interestingly, dsp1-1 dsp4-1 double mutants exhibit complete male sterility and reduced pre-snRNA transcription and 39-end maturation, unlike dsp1-1 or dsp4-1. In addition, Pol II occupancy at snRNA loci is lower in dsp1-1 dsp4-1 than in either single mutant. We also detected miscleaved pre-snRNAs in dsp1-1 dsp4-1, but not in dsp1-1 or dsp4-1. Taken together, these data reveal that DSP1 and DSP4 function is essential for pollen development, and that the two cooperatively promote pre-snRNA transcription and 39-end processing efficiency and accuracy. Small nuclear RNAs (snRNAs; Hadjiolov et al., 1966), a class of noncoding RNAs, are the basal components of the spliceosome and play essential roles in pre-mRNA splicing (Black et al., 1985; Ray et al., 1997; Guo et al., 2009). Their biogenesis involves transcription and subsequent processing steps. In Arabidopsis (Arabidopsis thaliana), the DNA-dependent RNA polymerase II (Pol II) synthesizes the primary snRNA transcripts (pre-snRNAs) U1, U2, U4, U5, and U12, but not U6, which is transcribed by Pol III (Carbon et al., 1987; Vankan and Filipowicz, 1988). Following transcription, pre-snRNAs are subjected to endonucleolytic cleavage at specific sites to remove the RNA fragment transcribed beyond the 39-end of mature snRNAs. In metazoans, snRNA 39-end maturation requires the Integrator Complex (INT; Baillat et al., 2005). INT contains at least 14 subunits (Baillat et al., 2005; Chen and Wagner, 2010), associates with the C-terminal domain of the largest subunit of Pol II, and depends on transcription for complex formation. It cotranscriptionally cleaves pre-snRNAs upstream of the required 39-box RNA motif (Uguen and Murphy, 2003, 2004; Baillat et al., 2005;Chen and Wagner, 2010). Integrator Subunit 11 (INT11) is a putative member of the metallob-lactamase (MBL)/b-CASP family of RNA endonucleases, is homologous to the cleavage and polyadenylation

Small nuclear RNAs (snRNAs) are the basal components of the spliceosome and play crucial roles in splicing. Their biogenesis is spatiotemporally regulated. However, related mechanisms are still poorly understood. Defective in snRNA processing (DSP1) is an essential component of the DSP1 complex that catalyzes plant snRNA 3′-end maturation by cotranscriptional endonucleolytic cleavage of the primary snRNA transcripts (presnRNAs). Here, we show that DSP1 is subjected to alternative splicing in pollens and embryos, resulting in two splicing variants, DSP1α and DSP1β. Unlike DSP1α, DSP1β is not required for presnRNA 3′-end cleavage. Rather, it competes with DSP1α for the interaction with CPSF73-I, the catalytic subunit of the DSP1 complex, which promotes efficient release of CPSF73-I and the DNA-dependent RNA polymerease II (Pol II) from the 3′ end of snRNA loci thereby facilitates snRNA transcription termination, resulting in increased snRNA levels in pollens. Taken together, this study uncovers a mechanism that spatially regulates snRNA accumulation.