Iskra Katic

Multicopy transgene arrays remain widely used in Caenorhabditis elegans research. It is usually assumed that they behave neutrally, not impacting the phenotype under investigation. Here, we reveal that a previously reported heterochronic extra-molt phenotype associated with myrf-1(mg412) depends on the presence of the integrated molting reporter mgIs49. Nanopore long-read sequencing shows that mgIs49 is a massive 8.8-Mb insertion—around 50% of the size of its host chromosome—which disrupts the prmt-9 gene. Both mgIs49 and another array, maIs105, cause dysregulation of the transcriptome and accumulation of reads mapping in sense-direction to transgene promoter sequences. We identify additional arrays exceeding 4 Mb and show that variable molting defects occur across different transgenic lines when combined with myrf-1(mg412), implicating array size or composition in the synthetic phenotype. Our results underscore the necessity of replacing multicopy reporters in developmental studies with single-copy insertions or endogenous tagging whenever possible.

Multicopy transgene arrays remain widely used in C. elegans research. It is usually assumed that they behave neutrally, not impacting the phenotype under investigation. Here, we reveal that a previously reported heterochronic extra-molt phenotype associated with myrf-1(mg412) depends on the presence of the integrated molting reporter mgIs49 . Nanopore long-read sequencing shows that mgIs49 is a massive 8.8-Mb insertion - around 50% of the size of its host chromosome - which disrupts the prmt-9 gene. Both mgIs49 and another array, maIs105 , cause dysregulation of the transcriptome and accumulation of reads mapping to the promoter sequences used as components of the array. We identify additional arrays exceeding 4 Mb and show that variable molting defects occur across different transgenic lines when combined with myrf-1(mg412) , implicating array size or composition in the synthetic phenotype. Our results underscore the necessity of replacing multicopy reporters in developmental studies with single-copy insertions or endogenous tagging whenever possible.

The segregation of the genome into accessible euchromatin and histone H3K9-methylated heterochromatin helps silence repetitive elements and tissue-specific genes. In Caenorhabditis elegans, MET-2, the homologue of mammalian SETDB1, catalyzes H3K9me1 and me2, yet like SETDB1, its regulation is enigmatic. Contrary to the cytosolic enrichment of overexpressed MET-2, we show that endogenous MET-2 is nuclear throughout development, forming perinuclear foci in a cell cycle–dependent manner. Mass spectrometry identified two cofactors that bind MET-2: LIN-65, a highly unstructured protein, and ARLE-14, a conserved GTPase effector. All three factors colocalize in heterochromatic foci. Ablation of lin-65, but not arle-14, mislocalizes and destabilizes MET-2, resulting in decreased H3K9 dimethylation, dispersion of heterochromatic foci, and derepression of MET-2 targets. Mutation of met-2 or lin-65 also disrupts the perinuclear anchoring of genomic heterochromatin. Loss of LIN-65, like that of MET-2, compromises temperature stress resistance and germline integrity, which are both linked to promiscuous repeat transcription and gene expression.