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

Abstract

In this study, we show that the haplo-enhancer effect of JIL-1 has the ability to counterbalance the haplo-suppressor effect of both Su(var)3-9 and Su(var)2-5 on position-effect variegation, providing evidence that a finely tuned balance between the levels of JIL-1 and the major heterochromatin components contributes to the regulation of gene expression.T HE essential JIL-1 histone H3S10 kinase (Jin et al. 1999;Wang et al. 2001) is a major regulator of chromatin structure (Deng et al. 2005(Deng et al. , 2008) that functions to maintain euchromatic domains while counteracting heterochromatization and gene silencing (Ebert et al. 2004;Lerach et al. 2006;Zhang et al. 2006;Bao et al. 2007). 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. 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 ( ). 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. This raises the question of whether JIL-1 dynamically interacts with the two other heterochromatin genes, Su(var)2-5 and Su(var)3-9, in regulating gene expression, as it does with Su (var)3-7.To answer this question, we explored the effect of various combinations of loss-of-function alleles of JIL-1 and Su(var) 3-9 or Su(var)2-5 on PEV caused by the P-element insertion line 118E-10 (Wallrath and Elgin 1995;Wallrath et al. 1996). 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-en...

The epigenetic H3S10 phosphorylation mark is required for counteracting heterochromatic spreading and gene silencing inDrosophila melanogaster

“…Therefore, these findings provide additional evidence that such parameters are probably independent of, and separate from, the mechanisms of classical heterochromatin assembly and gene silencing. This hypothesis is supported by experiments probing for dynamic interactions between loss-of-function alleles of JIL-1 and Su(var)3-9, Su(var)3-7 or Su(var)2-5 using PEV assays (Deng et al, 2010;Wang et al, 2011), where a direct antagonistic and counterbalancing effect on gene expression between JIL-1 and all three heterochromatic factors has been demonstrated.…”

“…PEV assays were performed as previously described Bao et al, 2007;Deng et al, 2010;Wang et al, 2011). In short, we generated flies expressing the various JIL-1 constructs in a background of the two PEV arrangements (118E-10 or w m4 ) by standard crossing.…”

“…Mutational analyses have shown that the JIL-1 gene is essential for viability (Wang et al, 2001;Zhang et al, 2003) and that a reduction in JIL-1 kinase activity leads to a global disruption of polytene chromosome morphology (Wang et al, 2001;Deng et al, 2005). Furthermore, genetic interaction assays with JIL-1 hypomorphic and null allelic combinations demonstrated that the JIL-1 protein can counterbalance the effect of the three major heterochromatin components Su(var)3-9, Su(var)3-7 and Su(var)2-5 (HP1a) on position-effect variegation (PEV) (Deng et al, 2010;Wang et al, 2011). Based on these observations, it has been proposed that the epigenetic H3S10 phosphorylation mark functions to counteract heterochromatic spreading and gene silencing in Drosophila melanogaster (Ebert et al, 2004;Zhang et al, 2006;Deng et al, 2007;Deng et al, 2010).…”

Chromatin-Associated Proteins HP1 and Mod(mdg4) Modify Y-Linked Regulatory Variation in the Drosophila Testis

“…As expected, we observed that the mutation in Su JIL-1 is a multidomain protein responsible for the phosphorylation of the histone H3, as well as maintenance of the structure of the polytene chromosomes in Drosophila. JIL-1 LoF alleles have a strong effect on PEV, and JIL-1; Su(var) 205 and JIL-1; Su(var)3-9 double mutants show enhanced variegation (Wang et al 2011). This indicates that reduced dosage of JIL-1 can counterbalance the typical suppressive effect of HP1 and Su(var)3-9 on PEV.…”

“…JIL-1 has histone kinase activity (Jin et al 1999;Rath et al 2006;Zhang et al 2006) and is involved in the negative regulation of chromatin silencing and in chromosome organization. Deficiency of JIL-1 leads to strong enhancer of variegation (white-eye phenotype), counteracting even the suppressor-of-variegation effect of HP1 and Su(var)3-9 (Wang et al 2011).…”