Mutations in Drosophila heat shock cognate 4 are enhancers of Polycomb - PubMed (original) (raw)

Mutations in Drosophila heat shock cognate 4 are enhancers of Polycomb

R Mollaaghababa et al. Proc Natl Acad Sci U S A. 2001.

Abstract

The homeotic genes controlling segment identity in Drosophila are repressed by the Polycomb group of genes (PcG) and are activated by genes of the trithorax group (trxG). An F(1) screen for dominant enhancers of Polycomb yielded a point mutation in the heat shock cognate gene, hsc4, along with mutations corresponding to several known PcG loci. The new mutation is a more potent enhancer of Polycomb phenotypes than an apparent null allele of hsc4 is, although even the null allele occasionally displays homeotic phenotypes associated with the PcG. Previous biochemical results had suggested that HSC4 might interact with BRAHMA, a trxG member. Further analyses now show that there is no physical or genetic interaction between HSC4 and the Brahma complex. HSC4 might be needed for the proper folding of a component of the Polycomb repression complex, or it may be a functional member of that complex.

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Figures

Figure 1

Figure 1

Polycomb enhancement phenotypes of_hsc4_ 54.1. (A) The abdominal cuticle of a wild-type female. (B) The abdominal cuticle of a 54.1/Pc 3 female. Note that the first abdominal tergite (marked 1t) is transformed to the character of the second abdominal segment. Likewise, the sixth abdominal tergite (6t) and sternite (6s) are transformed to the character of the seventh abdominal segment. The seventh tergite and sternite are almost completely removed, partially transformed to the eighth abdominal segment. (C) The wing from a_54.1/Pc_ 3 adult. Patches of wing tissue are transformed to the character of haltere, especially along the posterior margin (arrowhead). (D–F) Legs from a_54.1/Pc_ 3 male. (D) The first thoracic leg, which shows a row of sex comb teeth (arrowhead) characteristic of the wild-type male foreleg. (E) The second thoracic leg. Note the complete row of ectopic sex comb teeth (arrowhead). (F) The third thoracic leg. There is a partial row of ectopic sex comb teeth (arrowhead).

Figure 2

Figure 2

Embryonic and larval phenotypes of_hsc4_ 54.1. (A) ABD-B staining of a stage 14 embryo obtained from crossing the_ru hsc4_ 54.1 sr e and ru h th st cu hsc4 54.1 stocks. The embryo was cut open along the dorsal midline and flattened. Note the ectopic expression of ABD-B, especially in the central nervous system, up through the second thoracic segment, or parasegment 4 (arrowhead). A bracket marks parasegment 10, the anterior limit of ABD-B expression in wild-type embryos. (B) The ventral cuticle of a ru h th st cu hsc4 54.1 /+ second instar larva. The denticle belts of the third, fourth, and fifth abdominal segments are marked. Most of the denticles of the fourth abdominal segment are missing (arrow).

Figure 3

Figure 3

HSC4 separation from the BRM complex in embryos. (A) Superose 6 gel filtration chromatography of embryo extracts, with fractions separated on Western blots and probed for BRM or HSC4. Lanes are designated by column fraction number or L, material loaded. Elution volumes of native molecular mass standards are indicated above. There is only a small amount of overlap between the BRM and HSC4 peaks. (B) BRM immunoprecipitates. An anti-HA mAb was used to immunoprecipitate extracts from wild-type control embryos (C) or embryos containing HA-epitope-tagged BRM protein (HA). Immunoprecipitation was from the pooled BRM peak fractions of a Superose 6 column. Western blotting of the starting material (total), supernatant (sup), and pellet samples revealed only trace amounts of HSC4 in both pellets, indicating no specific association with BRM.

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