Figure 5. A model for formation of the DAZ.
A. In a wild-type strain, each terminus arm flanking dif (the black and white square) harbours a series of identically oriented polar elements (the white and black arrows for the left and the right terminus arms respectively). The DAZ is the region where these series converge.
B. Effects of chromosome rearrangements on the DAZ. The end-points of the various rearrangements are indicated by vertical arrows, and deleted segments are indicated by dashed lines.
B1. Deletions in one of the terminus arms eliminate polar elements, but dif still lies in the convergence region of the remaining elements. Strains harbouring such deletions are Dif+. This is the case for all deletions on the same side of dif, such as the zdc310–zdc330 segments (data not shown) or deletions from Cornet et al. (1996).
B2. In deletions removing dif, the end-points of the deletion lie systematically at the convergence region. Reinsertion of dif at the end-points of the deletion yields Dif+ strains. Examples come from Cornet et al. (1996) and from deletions reported in Fig. 3.
B3. Inversions including dif reshuffle the left and right terminus arms and their polar elements, but dif still lies at the convergence region. Strains harbouring these inversions are Dif+. For example, see Fig. 4A, the inversion of the zda192–zdd347 segment.
B4. Inversion of segments entirely contained in one terminus arm (i.e. not containing dif) displace the convergence region to the end-point of the inversion furthest from dif. Thus, dif no longer lies in the competence region. Strains harbouring such inversions are Dif− but regain a Dif+ phenotype when dif is transplaced to the new convergence region. Inversion of the zda192–zdc338 segment is an example (Fig. 4A).
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