Alternative snRNAs in early zebrafish embryogenesissnRNA, spliceosome, dual translation machinery, embryogenesis 2 1 ABSTRACT 2 Splicing removes intronic RNA sequences are removed from pre-mRNA molecules and 3 enables, by alternative splicing, the generation of multiple unique RNA molecules from a 4 single gene. As such, splicing is an essential part of the whole translation system of a cell. 5 The spliceosome is a ribonucleoprotein complex in which five small nuclear RNAs (snRNAs) 6 are involved; U1, U2, U4, U5, and U6. For each of these snRNAs there are variant gene 7 copies present in a genome. Furthermore, in many eukaryotic species there is an 8 alternative, minor spliceosome that can splice a small number of specific introns. As we 9 previously discovered an embryogenesis-specific ribosomal system in zebrafish early 10 embryogenesis based on variant rRNA and snoRNA expression, we hypothesized that there 11 may also be an embryogenesis-specific spliceosome. An inventory of zebrafish snRNA genes 12 revealed clustered and dispersed loci for all but U2 major snRNAs. For each minor 13 spliceosome snRNA, just one gene locus was found. Since complete snRNA molecules are 14 hard to sequence, we employed a combined PCR-sequencing approach to measure the 15 individual snRNA-variant presence. Analysis of egg and male-adult samples revealed 16 embryogenesis-specific and somatic-specific variants for each major snRNA. These variants 17 have substantial sequence differences, yet none in their mRNA binding sites. Given that 18 many of the sequence differences are found in loop structures indicate possible alternative 19 protein binding. Altogether, with this study we established that the spliceosome is also an 20 element of the embryogenesis-specific translation system in zebrafish. 21 22 23 24 25 26 27 3 28 INTRODUCTION29 Alternative splicing is fundamental for gene regulation and the generation of different 30 transcripts and/or proteins from an individual gene in eukaryotes (1). Splicing is executed by 31 the spliceosome and removes intronic sequences from pre-mRNA during the maturation 32 process in which the exonic sequences eventually form the mRNA (2,3).The spliceosome is a 33 molecular complex formed by hundreds of proteins and five essential small-nuclear RNAs 34 (snRNAs) that are typically located in the nucleus. The size of these small RNA molecules 35 ranges from 118 nucleotides (nt) to 191 nt. As they are uracil rich, they are called U1, U2, 36 U4, U5 and U6 snRNAs. Next to this major spliceosome, a minor (or U12 dependent) 37 spliceosome exists in many eukaryotic species, which is involved in the splicing of a relative 38 small number of specific introns (4). The snRNAs involved in the minor spliceosome are: 39 U11, U12, U4atac, and U6atac, completed by the U5 from the major spliceosome (4).40 As splicing is at the core of the cellular translation system, the sequences of the involved 41 snRNA are highly conserved across species. At the same time, many non-canonical variants 42 and gene copies of the major s...