Fibrillarin methylates H2A in RNA polymerase I trans-active promoters in Brassica oleracea

Abstract

Fibrillarin is a well conserved methyltransferase involved in several if not all of the more than 100 methylations sites in rRNA which are essential for proper ribosome function. It is mainly localized in the nucleoli and Cajal bodies inside the cell nucleus where it exerts most of its functions. In plants, fibrillarin binds directly the guide RNA together with Nop56, Nop58, and 15.5ka proteins to form a snoRNP complex that selects the sites to be methylated in pre-processing of ribosomal RNA. Recently, the yeast counterpart NOP1 was found to methylate histone H2A in the nucleolar regions. Here we show that plant fibrillarin can also methylate histone H2A. In Brassica floral meristem cells the methylated histone H2A is mainly localized in the nucleolus but unlike yeast or human cells it also localize in the periphery of the nucleus. In specialized transport cells the pattern is altered and it exhibits a more diffuse staining in the nucleus for methylated histone H2A as well as for fibrillarin. Here we also show that plant fibrillarin is capable of interacting with H2A and carry out its methylation in the rDNA promoter.

Ribosome Biogenesis in Plants: From Functional 45S Ribosomal DNA Organization to Ribosome Assembly Factors

“…Remarkably, it appears that the role of FIBRILLARIN is not limited to the methylation of rRNA. Indeed, FIBRILLARIN methylates histone H2A on rRNA gene promoters in both humans and Brassica species (Tessarz et al, 2014;Loza-Muller et al, 2015). Furthermore, a ribonuclease activity was demonstrated for the GAR domain of Arabidopsis FIB2 but not for that of FIB1 (Rodriguez-Corona et al, 2017).…”

ELF18‐INDUCED LONG NONCODING RNA 1 evicts fibrillarin from mediator subunit to enhance PATHOGENESIS‐RELATED GENE 1 (PR1) expression

“…Several papers reported that the formation of ribonucleoproteins between plant fibrillarin and viral RNAs is critical to protect the latter from the host defense system and to ensure systemic viral infection in plants (Kim et al ., ; Chang et al ., ). Arabidopsis FIB2 is primarily located in the nucleolus, but also in the nucleoplasm (Loza‐Muller et al ., ). Its presence in the nucleoplasm is consistent with the observation of Bäckström et al .…”

“…Being located only in the nucleolus, the function of NOP1 is restricted to the rDNA locus (Tessarz et al, 2014). By contrast, the Arabidopsis FIB2 is also found outside of the nucleolus, suggesting functions beyond the rDNA locus; moreover, it also has a conserved methyltransferase domain (Loza-Muller et al, 2015). It is known that the FACT complex interacts with all three RNA polymerases and facilitates transcription by disrupting nucleosomes in the path of the polymerase and increasing the redeposition of nucleosomes in the wake of transcribing polymerase (Hammond et al, 2017).…”

“…A methyltransferase domain is located through the central and alphahelix-rich region ( Fig. 2a) (Loza-Muller et al, 2015). To investigate the function of FIB2 in plant innate immunity, we obtained T-DNA insertion mutants' alleles (fib2-1; SALK_022153 and fib2-2; SALK_093373) from the SALK collection ( Fig.…”

Linking RNA Processing and Function

“…FBL is well-known as an rRNA 2′-O-methyltransferase in the snoRNP particles that participate in pre-rRNA processing (Tollervey et al 1991;Tollervey et al 1993;Tafforeau et al 2013) and Pol I transcription (Tessarz et al 2014;Loza-Muller et al 2015). Its amino-terminus contains a glycine-and arginine-rich (GAR) domain highly enriched with intrinsically disordered regions (IDRs) that are required for Xenopus laevis FBL phase separation (Feric et al 2016); its carboxyl terminus contains an RNA binding domain (RBD) and the α domain for methyltransferase activity (MD domain).…”