The catalytic-dead Pcif1 regulates gene expression and fertility in Drosophila.

Eukaryotic mRNAs are modified at the 5' end with a methylated guanosine (m7G) that is attached to the transcription start site (TSS) nucleotide. The TSS nucleotide is 2'-O-methylated (Nm) by CMTR1 in organisms ranging from insects to human. In mammals, the TSS adenosine can be further N 6 -methylated by RNA polymerase II phosphorylated CTD-interacting factor 1 (PCIF1) to create m6Am. Curiously, the fly ortholog of mammalian PCIF1 is demonstrated to be catalytic-dead, and its functions are not known. Here, we show that Pcif1 mutant flies display a reduced fertility which is particularly marked in females. Deep sequencing analysis of Pcif1 mutant ovaries revealed transcriptome changes with a notable increase in expression of genes belonging to the mitochondrial ATP synthetase complex. Furthermore, the Pcif1 protein is distributed along euchromatic regions of polytene chromosomes, and the Pcif1 mutation behaved as a modifier of position-effect-variegation (PEV) suppressing the heterochromatin-dependent silencing of the white gene. Similar or stronger changes in the transcriptome and PEV phenotype were observed in flies that expressed a cytosolic version of Pcif1. These results point to a nuclear cotranscriptional gene regulatory role for the catalytic-dead fly Pcif1 that is probably based on its conserved ability to interact with the RNA polymerase II carboxy-terminal domain.

The Mammalian Cap-Specific m6Am RNA Methyltransferase PCIF1 Regulates Transcript Levels in Mouse Tissues.

The 5' end of eukaryotic mRNAs is protected by the m7G-cap structure. The transcription start site nucleotide is ribose methylated (Nm) in many eukaryotes, whereas an adenosine at this position is further methylated at the N6 position (m6A) by the mammalian Phosphorylated C-terminal domain (CTD)-interacting Factor 1 (PCIF1) to generate m6Am. Here, we show that although the loss of cap-specific m6Am in mice does not affect viability or fertility, the Pcif1 mutants display reduced body weight. Transcriptome analyses of mutant mouse tissues support a role for the cap-specific m6Am modification in stabilizing transcripts. In contrast, the Drosophila Pcif1 is catalytically dead, but like its mammalian counterpart, it retains the ability to associate with the Ser5-phosphorylated CTD of RNA polymerase II (RNA Pol II). Finally, we show that the Trypanosoma Pcif1 is an m6Am methylase that contributes to the N6,N6,2'-O-trimethyladenosine (m62Am) in the hypermethylated cap4 structure of trypanosomatids. Thus, PCIF1 has evolved to function in catalytic and non-catalytic roles.