Figure 10

Model of interaction between HP1a and the PcG/trxG proteins. In early wild-type embryos, HP1a recruits the PcG/trxG proteins for H3K27me3/H3K4me3 deposition at promoters. H3K9me3 levels are also set. As constitutive heterochromatin is formed, HP1a is also recruited to these sites (blue circles), establishing the normal euchromatin promoter/heterochromatin distributions of PcG/trxG proteins and HP1a and their respective marks. These levels are maintained into the larval stage. When HP1a from the mother is reduced (middle), the embryo has reduced H3K27me3/H3K4me3 at promoters triggering an increase in H3K9me3 at promoters and heterochromatin (black dots). As constitutive heterochromatin forms, the insufficient maternal HP1a results in a derepression of repeated sequences. Pericentric H3K9me3 levels are reduced and telomeric regions are also affected. We propose this reduced HP1a stimulates the PcG chromatin system to compensate, elevating H3K27me3 levels. By third instar larvae, this causes an increase in H3K27me3 at promoters compared to wild-type, while the H3K9me3 levels are decreased. When the HP1a mutation is paternally received (bottom), maternal HP1a levels are normal in the early embryo. Constitutive heterochromatin can be formed normally without triggering the PcG chromatin system to compensate. All the silencing mark levels are presumed to be normal. However, as development proceeds, the levels of HP1a become insufficient due to the mutation, leading to a decrease in H3K27me3 as well as H3K9me3 at gene promoters. Repeated elements are less affected perhaps because their state was established early when maternal HP1a levels were normal. Size and weight of arrows, and color intensity depicts strength of effects or activity. K9 HMT depicts H3K9 histone methyltransferase (Setdb1 or Su(var)3-9 depending on the location). HP1, heterochromatin protein 1; H3K4me3, trimethylated histone H3 at lysine 4; H3K9me3, trimethylated histone H3 at lysine 9; H3K27me3, trimethylated histone H3 at lysine 27.