Abstract
Nopp140, often called the nucleolar and Cajal body phosphoprotein (NOLC1), is an evolutionarily conserved chaperone for the transcription and processing of rRNA during ribosome subunit assembly. Metazoan Nopp140 contains an amino terminal LisH dimerization domain and a highly conserved carboxyl domain. A large central domain consists of alternating basic and acidic motifs of low sequence complexity. Orthologous versions of Nopp140 contain variable numbers of repeating basic–acidic units. While vertebrate Nopp140 genes use multiple exons to encode the central domain, the Nopp140 gene in Drosophila uses exclusively exon 2 to encode the central domain. Here, we define three overlapping repeat sequence patterns (P, P′, and P″) within the central domain of D. melanogaster Nopp140. These repeat patterns are poorly conserved in other Drosophila species. We also describe a length polymorphism in exon 2 that pertains specifically to the P′ pattern in D. melanogaster. The pattern displays either two or three 96 base pair repeats, respectively, referred to as Nopp140-Short and Nopp140-Long. Fly lines homozygous for one or the other allele, or heterozygous for both alleles, show no discernible phenotypes. PCR characterization of the long and short alleles shows a poorly defined, artifactual bias toward amplifying the long allele over the short allele. The significance of this polymorphism will be in discerning the largely unknown properties of Nopp140’s large central domain in rDNA transcription and ribosome biogenesis.
Highlights
The nucleolar phosphoprotein of 140 kDa (Nopp140) is a ribosome assembly factor that locates within the dense fibrillar component of nucleoli and within nuclear Cajal bodies (CBs) (Meier and Blobel 1990, 1992)
When comparing the second exon of Nopp140 from Drosophila melanogaster with Nopp140 genes from other Drosophila species (D. simulans, D. yakuba, D. erecta, D. ananassae, D. pseudoobscura, D. virilis, and D. grimshawi), we found that only D. melanogaster carries the repeating patterns P′′, P′ and P
We considered the possibility of an in vivo gene conversion event occurring in the heterozygous flies whereby an insertion of a 96 bp repeat into the Nopp140-Short alleles would increase the abundance of the Nopp140-Long alleles
Summary
The nucleolar phosphoprotein of 140 kDa (Nopp140) is a ribosome assembly factor that locates within the dense fibrillar component of nucleoli and within nuclear Cajal bodies (CBs) (Meier and Blobel 1990, 1992). Subsequent work showed that Nopp140 associates with C/D box small nucleolar ribonucleoprotein (snoRNPs) that guide nucleotide-specific 2′-O-methylation of the pre-rRNA, and with H/ACA box snoRNPs that direct site-specific pseudouridylation of pre-rRNA and snRNAs (Yang et al 2000). These interactions with Nopp140 may initiate within CBs as Nopp140 is thought to shuttle these snoRNPs from CBs to nucleoli (Isaac et al 1998; Yang et al 2000; Wang et al 2002; Meier 2005; Lo et al 2006; He and DiMario 2011). Nopp140 interacts with RNA Pol I to regulate rRNA transcription, perhaps linking RNA Pol I transcription with pre-rRNA processing (Chen et al 1999; Meier 2005)
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