Selective Factor-1 Research Articles

Impaired function of rDNA transcription initiation machinery leads to derepression of ribosomal genes with insertions of R2 retrotransposon.

Eukaryotic genomes harbor hundreds of rRNA genes, many of which are transcriptionally silent. However, little is known about selective regulation of individual rDNA units. In Drosophila melanogaster, some rDNA repeats contain insertions of the R2 retrotransposon, which is capable to be transcribed only as part of pre-rRNA molecules. rDNA units with R2 insertions are usually inactivated, although R2 expression may be beneficial in cells with decreased rDNA copy number. Here we found that R2-inserted rDNA units are enriched with HP1a and H3K9me3 repressive mark, whereas disruption of the heterochromatin components slightly affects their silencing in ovarian germ cells. Surprisingly, we observed a dramatic upregulation of R2-inserted rRNA genes in ovaries lacking Udd (Under-developed) or other subunits (TAF1b and TAF1c-like) of the SL1-like complex, which is homologues to mammalian Selective factor 1 (SL1) involved in rDNA transcription initiation. Derepression of rRNA genes with R2 insertions was accompanied by a reduction of H3K9me3 and HP1a enrichment. We suggest that the impairment of the SL1-like complex affects a mechanism of selective activation of intact rDNA units which competes with heterochromatin formation. We also propose that R2 derepression may serve as an adaptive response to compromised rRNA synthesis.

Nucleolar localization signal and histone methylation reader function is required for SPIN1 to promote rRNA gene expression

Spindlin1 (SPIN1), a histone modification reader protein, was enriched in the cell nucleolus and facilitated rRNA expression. However, how SPIN1 localizes to the nucleolus and its functional role in rRNA gene expression remain unresolved. Here, we identified a nucleolar localization signal in the N-terminal region of SPIN1 that is essential for its enrichment and function in the nucleolus. We also discovered that, in addition to its H3K4me3 recognizing activity, the H3R8me2a-recognizing capacity of SPIN1 is also indispensable for stimulating rRNA expression. Chromatin immunoprecipitation results indicated that SPIN1 is required for the association or assembly of selective factor 1 (SL1) complex, probably facilitating the initiation of rDNA transcription through its H3 K4me3-R8me2a reader function.