crocodile

Transcriptional Regulation

The anterior, the dorsoventral and terminal systems, are required for the activation of crocodile expression and for the spatial control of the anterior cap domain, while the posterior system is not required for the regulation of the croc expression pattern. In the absence of bicoid activity, croc fails to be expressed in the anterior cap domain. Conversely, an increase in bcd activity in embryos leads to an expansion of croc expression toward the posterior. However, this expansion is one-sided with respect to the dorsoventral axis of the embryo. Thus, although Bcd acts in a concentration-dependent manner, croc expression can only be expanded ventrally in the absence of dorsal activity. In fact, the lack of dorsal activity causes a strong reduction of the croc expression domain to a single spot, corresponding in position to the peak of bicoid activity at the anterior pole. Conversely, Dorsal activity along the entire dorsoventral axis, as in embryos laid by Toll mutant females, causes an expansion of the croc expression domain towards the dorsal-most position. In embryos lacking torso activity, croc expression is abolished in the dorsal region. However, if tor is activated ectopically due to a dominant tor mutation, the croc expression domains are expanded significantly on the ventral side. Thus Dorsal postively regulates croc and Bcd requires Dl to set the spatial limit of the croc anterior expression domain. Mutations in the targets of Bicoid (orthodenticle, tailless and huckebein) and targets of Dorsal (twist, snail and zerknüllt )fail to alter croc expression. (Häcker, 1995).

Targets of Activity

The expression patterns of engrailed and wingless are altered in a single position in croc mutant embryos. Within the clypeolabral segment anlage, en expression is absent and the adjacent wg expression domain is significantly expanded and covers the area normally expressing en in addition to wg's normal expresion domain. Wild-type croc activity is therefore required for the control of segment polarity gene expression within a single segment equivalent, i.e. the clypeolabrum, the anterior-most segment equivalent of the head anlage of the embryo (Häcker, 1995).

Target analysis of the Croc forkhead domain was analyzed using a PCR/band shift support selection procedure from a mixture of deoxyoligonucleotides being degenerate at 16 consecutive postions. Approximately 90% of the selected sequenced targets share a 7 bp consensum motif consisting of A/GTAAAT/CA (Häcker, 1995).

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