BIOLOGICAL OVERVIEW

odd-paired appears to be an oddity among the pair-rule transcription factors. During early development of the embryo, odd-paired is not found in a pair-rule, strongly zebra-striped expression pattern, as are other pair-rule genes. Instead, it is transcribed ubiquitously in the central 60% of the embryo. Only later does it assume a banded appearance, in this case, 14 weakly staining zebra stripes. Since all other pair-rule proteins appear in only 7 stripes, OPA is indeed an anomaly. Its classification as a pair-rule gene arises from its mutant phenotype and not its expression pattern (Benedyk, 1994).

Despite its peculiarities, opa functions like other pair-rule genes, and is involved in the induction of wingless into the posterior compartments of parasegments. Because opa's effects are localized, while its expression is not, the positive regulation of wingless must be permissive and not instructive. opa doesn't signal location but simply gives permission. opa appears to act in conjuction with other transcription factors that provide positional signals.

One odd-paired function should be highlighted because it takes place in the mesoderm, rather than the ectoderm. opa is required for the formation of midgut constrictions. It is expressed ubiquitously in segmented regions of both ectoderm and mesoderm. This expression ceases and later reinitiates in restricted portions of the mesoderm. This expression is positively regulated by homeotic genes Antennapedia and abdominal-A, and negatively regulated by Ultrabithorax and decapentaplegic in opa negative areas (Cimbora, 1995).

Runt and Opa cooperate to activate slp1 transcription

A crucial step in generating the segmented body plan in Drosophila is establishing stripes of expression of several key segment-polarity genes, one stripe for each parasegment, in the blastoderm stage embryo. It is well established that these patterns are generated in response to regulation by the transcription factors encoded by the pair-rule segmentation genes. However, the full set of positional cues that drive expression in either the odd- or even-numbered parasegments has not been defined for any of the segment-polarity genes. Among the complications for dissecting the pair-rule to segment-polarity transition are the regulatory interactions between the different pair-rule genes. An ectopic expression system that allows for quantitative manipulation of expression levels was used to probe the role of the primary pair-rule transcription factor Runt in segment-polarity gene regulation. These experiments identify sloppy paired 1 (slp1), most appropriately classified as segment polarity genes, as a gene that is activated and repressed by Runt in a simple combinatorial parasegment-dependent manner. The combination of Runt and Odd-paired (Opa) is both necessary and sufficient for slp1 activation in all somatic blastoderm nuclei that do not express the Fushi tarazu (Ftz) transcription factor (see The pair-rule to segment-polarity transition). By contrast, the specific combination of Runt + Ftz is sufficient for slp1 repression in all blastoderm nuclei. Furthermore Ftz is found to modulate the Runt-dependent regulation of the segment-polarity genes wingless (wg) and engrailed (en). However, in the case of en the combination of Runt + Ftz gives activation. The contrasting responses of different downstream targets to Runt in the presence or absence of Ftz is thus central to the combinatorial logic of the pair-rule to segment-polarity transition. The unique and simple rules for slp1 regulation make this an attractive target for dissecting the molecular mechanisms of Runt-dependent regulation (Swantek, 2004; full text of article).

A somewhat surprising result from these experiments is that slp1 expression in odd-numbered parasegments is lost in opa mutant embryos (see Runt and Opa cooperate to activate slp1 transcription). The importance of Opa is surprising since expression of other segment-polarity genes is reduced, but not eliminated in opa mutants (Benedyk, 1994; Cimbora, 1995). Moreover, Opa is expressed at uniform levels throughout the pre-segmental region of the embryo, and thus does not provide positional information that defines the placement of slp1 stripes relative to other pair-rule transcription factors. The odd-numbered slp1 stripes require Runt, and are interpreted to expand in response to ectopic Runt. The requirement for Opa in this Runt-dependent activation was tested by examining slp1 expression in embryos that have high levels of NGT-driven (NGT stands for nanos-GAL4-tubulin) Runt and that are also mutant for opa. Expression of slp1 within the pre-segmental region is lost in these embryos. This result corroborates the interpretation that the expanded slp1 stripes produced by NGT-driven Runt correspond to the odd-numbered stripes and further confirms the importance of Opa for Runt-dependent activation (Swantek, 2004).

A useful feature of the GAL4 expression system is that expression levels can be varied by changing the strengths of either the GAL4 driver, or the responding UAS transgene. Advantage of this feature was taken to further investigate the relative roles of Runt and Opa in slp1 regulation by generating a co-expression matrix with a panel of different UAS-runt and UAS-opa lines. Increasing the level of Opa in embryos with the same low level of NGT-driven Runt alters slp1 in a manner similar to that obtained by increasing Runt alone. Thus, Opa potentiates Runt-dependent regulation in a concentration-dependent manner. Concentration-dependent effects of Opa are also observed at both intermediate and high levels of NGT-driven Runt. In order to interpret these changes, it is useful to first consider the relatively simple, yet striking response of slp1 to high levels of both Runt and Opa. In these embryos, slp1 is expressed throughout the anterior head region and is nearly uniformly repressed throughout the pre-segmental region of the embryo. The anterior activation is particularly informative since none of the other pair-rule or segment-polarity gene shows this response to Runt and Opa. Thus, anterior activation of slp1 by Runt and Opa occurs in the absence of regulatory inputs from other segmentation genes. It is notable that anterior activation can be triggered either by increasing the level of Runt in embryos with constant intermediate levels of Opa, or by increasing the levels of Opa in embryos with constant intermediate levels of Runt. The observation that Runt and Opa are both obligatory for anterior activation, coupled with this mutual dose-dependent cooperation strongly suggests that these two factors function together in a concentration-dependent complex to activate slp1transcription (Swantek, 2004).

The other notable response to high levels of Runt and Opa is the nearly complete repression of slp1 throughout the presegmental region of the embryo. slp1 and ftz are expressed in complementary patterns in embryos with high uniform levels of Runt. Examination of the response of ftz to the co-expression of Runt and Opa indicates a perfect correlation between the elimination of slp1 and the expansion of ftz. These observations indicate that Opa potentiates the ability of Runt to activate ftz. Moreover, these results strongly suggest that Ftz plays a key role in slp1 repression (Swantek, 2004).

GENE STRUCTURE

cDNA clone length - 2959

Bases in 5' UTR -293

Exons - four

Bases in 3' UTR - 820

PROTEIN STRUCTURE

Amino Acids - 609

Structural Domains

opa encodes a zinc finger protein with five fingers homologous to those of the Drosophila segment polarity gene cubitus interruptus, the human glioblastoma gene GLI and the C. elegans sex determination gene tra-1 (Benedyk, 1994).

date revised: 20 January 2007

Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.

The Interactive Fly resides on the
Society for Developmental Biology's Web server.