Figure 5.
A model of pair-rule gene interactions that position en stripes. Regulatory interactions are represented by arrows (activation) or T-shapes (repression). Anterior is to the left. (A) The anterior and posterior extents of initial expression patterns are represented by colored boxes. Refined patterns that are either activated later or result from subsequent interactions among pair-rule genes are shown either as a heavily outlined box or (in the cases of runt and eve) as a box at a slightly lower position in the diagram (for eve, these “minor” stripes remain weak relative to the thick-outlined late eve stripes). The initial patterns of the primary pair-rule genes hairy, runt, and eve are largely determined by the concentrations of the gap proteins, and ftz also has a strong input from gap proteins. Mutually repressive interactions between hairy and runt further contribute to the formation of their complementary patterns, which are diagrammed here. Regulatory interactions that point downward in the diagram contribute to the initial patterns of expression of downstream genes, while those that point upward generally produce a subsequent refinement of the initial pattern or regulate a part of the pattern that appears later. The experimental justification for each interaction is listed in Table 1, with the exception of number 10, which is as follows: in runt nulls, where eve expression persists abnormally, odd comes on in broad stripes in the eve domain in place of slp (Fig. 2); coupled with the fact that in eve nulls, slp is expressed throughout the eve domain (Fujioka et al., 1995), this suggests that late eve expression is normally responsible for setting the anterior border of secondary slp expression; this function may be taken over by odd as odd represses eve (see 6b, Table 1), and odd may also help to limit the posterior border of odd-numbered en stripes at this stage (Saulier-Le Dréan et al., 1998). Interaction number 4 is shown as a dotted line because, as described more fully in the text, odd expression is lost from the ftz domains in runt nulls as well as in runt, slp double mutants, so that odd may be primarily responsible for keeping en and late eve from expanding anteriorly in slp mutants, while slp may be solely responsible for setting this border in the wild type. Implicit in a number of these regulatory relationships is the fact that an effect of one gene on another may change during the course of refinement of pair-rule gene patterns, and similarly, that an effect in odd-numbered PSs does not imply the corresponding effect in even-numbered PSs, and vice versa. Such complexities can be explained by the existence of distinct regulatory elements in a gene, which drive expression of different aspects of its pattern and respond to distinct regulatory inputs. This is the case for the regulation of eve. In other cases, such as for runt, it may not be possible to dissect the cis-acting sequences into distinct elements (Klingler et al., 1996), and such complexities may be due to multiple factors acting combinatorially through common or overlapping elements. (B) Regulatory interactions can account for the well-established concentration-dependent positioning of PS boundaries by Eve and Ftz (see text for appropriate references). In the anterior half of each odd-numbered PS, the early eve stripe provides a concentration gradient of Eve protein just before and during cellularization of the blastoderm (shown as a blue curve at the top). Reduced Eve activity is represented by the yellow curve; other expression patterns are represented by colored boxes, with altered patterns that result from either decreased eve activity, manifested primarily at the left PS boundary in the diagram, or increased ftz activity, manifested primarily at the right PS boundary, represented by boxes offset below the wild-type patterns. The odd-numbered en stripe is activated by Prd and repressed by Slp, and both prd and slp are repressed by Eve. A high level of Eve is required to repress prd (Fujioka et al., 1995; Manoukian and Krause, 1992), which is activated earlier than slp in the trunk region, while a low level of Eve suffices to repress slp. The anterior border of this en stripe is positioned by the posterior edge of the slp stripe, while its posterior border is positioned by the posterior edge of prd expression, both of which are sensitive to the genetic dose of eve (Fujioka et al., 1999) and to the level of Eve’s repressor activity (Fujioka et al., 2002; Kobayashi et al., 2001). The net result of these interactions is that the positions of both the anterior and posterior borders of the odd-numbered en stripe respond to changes in the level of Eve, with lower Eve levels resulting in reduced-width odd-numbered PSs (and a boundary at the dotted yellow line), and therefore expanded even-numbered PSs. The positions of the even-numbered en stripes are particularly sensitive to the concentration of Ftz. Ftz activates these en stripes, while Odd represses them, setting the anterior and posterior borders, respectively. Higher levels of Ftz (or increased stability of Ftz) result in auto-activation over a wider region (Hiromi and Gehring, 1987; Ish-Horowicz et al., 1989), but also cause an anterior expansion of odd stripes. Eve represses odd at low concentrations, and is also capable of repressing ftz at higher concentrations (DiNardo and O’Farrell, 1987; Fujioka et al., 1995; Manoukian and Krause, 1992), while Odd causes subsequent downregulation of ftz expression. This combination of interactions allows the positions of the even-numbered en stripes to move anteriorly in response to higher Ftz activity, expanding the even-numbered PSs (with a boundary at the dotted green line) at the expense of the odd-numbered ones. Reduced levels of Ftz function or increased levels of Eve function produce the opposite effects of those described above, and mechanisms that are the converse of those described can account for these effects. The positions of the wg stripes move in conjunction with those of the en stripes. At the anterior boundaries of the odd-numbered PSs, wg is repressed by both En (Heemskerk et al., 1991) and late Eve expression (Ingham et al., 1988; Manoukian and Krause, 1992), which shares an anterior border with en (Lawrence et al., 1987). At the anterior boundaries of the even-numbered PSs, wg is repressed by ftz and en (Heemskerk et al., 1991; Ingham et al., 1988; Ish-Horowicz et al., 1989), so that in each case, wg expression is activated just anterior to that of en.