Fig. 9. A model for assembly of the Aly-class meiotic-arrest proteins at target promoters. (A) Normal assembly of an aly-class gene product complex is regulated at several steps. (1) Aly-Comr interaction facilitates their nuclear translocation (or possibly prevents nuclear export). Tomb, Topi and Achi/Vis proteins localise constitutively to the nucleus, and can bind with low affinity to target promoters. (2) Nuclear Aly and Comr bind to and stabilise Tomb, then interact with Topi and Achi/Vis to facilitate cooperative DNA binding. (3) Transcriptional activation requires tight association of all five components with DNA. (B) In comr (or aly) spermatocytes, Aly (or Comr) remains cytoplasmic, Tomb protein is destablised and Topi and Achi/Vis only weakly interact with DNA; transcription is not activated. (C) In tomb mutants, Aly and Comr are stable in the nucleus, but cannot promote Topi and Achi/Vis association with DNA; transcription is not activated. (D) In achi/vis (or topi) mutants, the complex is not efficiently associated with DNA; transcription is not activated.