The fission yeast methyl phosphate capping enzyme Bmc1 guides 2'-O-methylation of the U6 snRNA

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 humans and fission yeast, this includes addition of a 5 {gamma}-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 U6 and identify and characterize a non-canonical snoRNA that guides this methylation. Further, we show that fission yeast strains deleted of Bmc1 or Pof8 show altered U6 snRNP assembly patterns, supporting a more general role for these factors in guiding noncoding RNP assembly beyond the telomerase RNP. These results are thus consistent with a novel role for Bmc1/MePCE family members in stimulating U6 post-transcriptional modifications.