PT  - JOURNAL ARTICLE
AU  - Jennifer Porat
AU  - Viktor A. Slat
AU  - Stephen D. Rader
AU  - Mark A. Bayfield
TI  - The fission yeast methylphosphate capping enzyme Bmc1/Bin3 promotes 2’-O-methylation of U6 and pre-mRNA splicing
AID  - 10.1101/2023.01.27.525755
DP  - 2023 Jan 01
TA  - bioRxiv
PG  - 2023.01.27.525755
4099  - http://biorxiv.org/content/early/2023/01/28/2023.01.27.525755.short
4100  - http://biorxiv.org/content/early/2023/01/28/2023.01.27.525755.full
AB  - Efficient splicing requires the tight coordination of dynamic spliceosomal RNAs and proteins. U6 is the only spliceosomal RNA transcribed by RNA Polymerase III and undergoes an extensive maturation process. In both humans and fission yeast, this includes addition of a 5’ γ-monomethyl phosphate cap by members of the Bin3/MePCE family. Previously, we have shown that the Bin3/MePCE homolog Bmc1 is recruited to the S. pombe telomerase holoenzyme by the LARP7 family protein Pof8, where it acts in a catalytic-independent manner to protect the telomerase RNA and facilitate holoenzyme assembly. Here, we show that Bmc1 and Pof8 also interact in a U6-containing snRNP. We demonstrate that Bmc1 and Pof8 promote 2’-O-methylation of the U6 internal stem loop and identify and characterize a non-canonical snoRNA that guides this methylation. Fission yeast strains deleted of Bmc1 show altered U4/U6 di-snRNP assembly patterns and impaired splicing at elevated temperatures. These results are thus consistent with a novel role for Bmc1/MePCE family members in stimulating U6 post-transcriptional modifications and promoting U6 snRNP assembly and splicing fidelity.Significance Statement The spliceosomal RNA U6 undergoes numerous processing and post-transcriptional modification steps before incorporation into the spliceosome. Here, we identify a new U6-containing complex in fission yeast that shares components with the telomerase holoenzyme, including the 5’ phospho-methyltransferase Bmc1. This complex promotes 2’-O-methylation of U6 and influences formation of the U4/U6 di-snRNP, and cells lacking Bmc1 show splicing defects at inefficiently spliced introns at elevated temperatures. Our results reveal a novel complex of proteins and RNA that cooperate to ensure splicing fidelity.Competing Interest StatementThe authors have declared no competing interest.