Fig. 4. HP1 functions downstream of SU(VAR)3-9. (A) The lac repeats show a low level of HP1 and H3 K9 methylation in the absence of lacI-HP1. Polytene chromosomes from stock 157.1 containing a reporter transgene with 256 copies of lac repeats were stained with antibodies to HP1 (green) and antibodies to H3 K9 di-methyl (red). Arrows indicate the locations of lac repeats. (B) Tethered HP1 does not intensify H3 K9 methylation. Strong staining is observed for the HP1 antibody, but not the H3 K9 di-methyl antibody, upon expression of lacI-HP1. (C) Expression of a Su(var)3-9-EGFP transgene leads to recruitment of SU(VAR)3-9-EGFP (upper row) and increased H3 K9 methylation (bottom row). (D) Silencing by tethered HP1 (stock 157.1) was unaffected in Su(var)3-9⁰⁶ hetero- and homozygotes as indicated by a white eye phenotype (middle and bottom rows). The homozygous Su(var)3-9⁰⁶ flies show a rough eye phenotype due to the roughest mutation used as a marker on the Su(var)3-9⁰⁶-bearing chromosome (bottom row). By contrast, Su(var)3-9⁰⁶ heterozygotes show suppression of white gene in In(1)w^m4, a stock carrying an inversion placing the white gene next to centric heterochromatin, and 118E-10 a stock carrying a hsp70-white P-element inserted within centric heterochromatin.

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