JIL-1 and Su(var)3-7 Interact Genetically and Counteract Each Other's Effect on Position-Effect Variegation in Drosophila

Abstract

The essential JIL-1 histone H3S10 kinase is a key regulator of chromatin structure that functions to maintain euchromatic domains while counteracting heterochromatization and gene silencing. In the absence of the JIL-1 kinase, two of the major heterochromatin markers H3K9me2 and HP1a spread in tandem to ectopic locations on the chromosome arms. Here we address the role of the third major heterochromatin component, the zinc-finger protein Su(var)3-7. We show that the lethality but not the chromosome morphology defects associated with the null JIL-1 phenotype to a large degree can be rescued by reducing the dose of the Su(var)3-7 gene and that Su(var)3-7 and JIL-1 loss-of-function mutations have an antagonistic and counterbalancing effect on position-effect variegation (PEV). Furthermore, we show that in the absence of JIL-1 kinase activity, Su(var)3-7 gets redistributed and upregulated on the chromosome arms. Reducing the dose of the Su(var)3-7 gene dramatically decreases this redistribution; however, the spreading of H3K9me2 to the chromosome arms was unaffected, strongly indicating that ectopic Su(var)3-9 activity is not a direct cause of lethality. These observations suggest a model where Su(var)3-7 functions as an effector downstream of Su(var)3-9 and H3K9 dimethylation in heterochromatic spreading and gene silencing that is normally counteracted by JIL-1 kinase activity.

A Balance Between Euchromatic (JIL-1) and Heterochromatic [SU(VAR)2-5 and SU(VAR)3-9] Factors Regulates Position-Effect Variegation inDrosophila

“…Interestingly, in other genetic interaction assays monitoring the lethality as well as the chromosome morphology defects associated with the null JIL-1 phenotype, only a reduction in the dose of the Su(var) 3-9 gene Deng et al 2007) rescued both phenotypes. In contrast, in the same assays a reduction of Su(var)3-7 rescued the lethality, but not the chromosome defects (Deng et al 2010), and no genetic interactions were detectable between JIL-1 and Su(var)2-5 ). Thus, these findings indicate that while Su(var)3-9 activity may be a major factor in the lethality and chromatinstructure perturbations associated with loss of the JIL-1 histone H3S10 kinase, these effects are likely to be uncoupled from HP1a and, to a lesser degree, from Su(var)3-7.…”

“…These observations suggested a model for a dynamic balance between euchromatin and heterochromatin (Ebert et al 2004;Zhang et al 2006;Deng et al 2010), where, as can be monitored in position-effect variegation (PEV) arrangements, the boundary between these two states is determined by antagonistic functions of a euchromatic regulator (JIL-1) and the major determinants of heterochromatin assembly, e.g., Su(var)3-9, HP1a, and Su(var)3-7 (for review see Weiler and Wakimoto 1995;Girton and Johansen 2008). In support of this model, Deng et al (2010) recently showed that Su(var)3-7 and JIL-1 loss-of-function mutations have an antagonistic and counterbalancing effect on gene expression using PEV assays; however, potential dynamic interactions between JIL-1 and the other two heterochromatin genes, Su(var)3-9 and Su(var)2-5, were not addressed in this study. Interestingly, in other genetic interaction assays monitoring the lethality as well as the chromosome morphology defects associated with the null JIL-1 phenotype, only a reduction in the dose of the Su(var) 3-9 gene Deng et al 2007) rescued both phenotypes.…”

“…Insertion of this P element (P[hsp26-pt, hsp70-w]) into euchromatic sites results in a uniform red-eye phenotype whereas insertion into a known heterochromatin region of the fourth chromosome results in a variegating eye phenotype (Cryderman et al 1998;Bao et al 2007) (Figures 1 and 2). It has been demonstrated that loss-of-function JIL-1 alleles can act as haplo-enhancers of PEV, resulting in increased silencing of gene expression (Deng et al 2010), whereas loci for structural components of heterochromatin …”

“…In the absence of the JIL-1 kinase, the major heterochromatin markers H3K9me2, HP1a [Su(var)2-5], and Su(var)3-7 spread to ectopic locations on the chromosome arms Deng et al 2007Deng et al , 2010. These observations suggested a model for a dynamic balance between euchromatin and heterochromatin (Ebert et al 2004;Zhang et al 2006;Deng et al 2010), where, as can be monitored in position-effect variegation (PEV) arrangements, the boundary between these two states is determined by antagonistic functions of a euchromatic regulator (JIL-1) and the major determinants of heterochromatin assembly, e.g., Su(var)3-9, HP1a, and Su(var)3-7 (for review see Weiler and Wakimoto 1995;Girton and Johansen 2008). In support of this model, Deng et al (2010) recently showed that Su(var)3-7 and JIL-1 loss-of-function mutations have an antagonistic and counterbalancing effect on gene expression using PEV assays; however, potential dynamic interactions between JIL-1 and the other two heterochromatin genes, Su(var)3-9 and Su(var)2-5, were not addressed in this study.…”

“…Insertion of this P element (P[hsp26-pt, hsp70-w]) into euchromatic sites results in a uniform red-eye phenotype whereas insertion into a known heterochromatin region of the fourth chromosome results in a variegating eye phenotype (Cryderman et al 1998;Bao et al 2007) (Figures 1 and 2). It has been demonstrated that loss-of-function JIL-1 alleles can act as haplo-enhancers of PEV, resulting in increased silencing of gene expression (Deng et al 2010), whereas loci for structural components of heterochromatin such as Su(var)3-9, Su(var)2-5, and Su(var)3-7 act as strong haplo-suppressors (Eissenberg et al 1990;Reuter et al 1990;Tschiersch et al 1994). In the experiments, the transgenic reporter line 118E-10 was crossed into JIL-1 z2 /1, Su(var)3-9 06 /1, and Su(var)2-5 05 /1 mutant backgrounds as well as into JIL-1 z2 /Su(var)3-9 06 and JIL-1 z2 /Su(var)2-5 05 doublemutant backgrounds.…”