Aqn

pole celts with oskar mRNA

FIGURE 18-17 The bicoid and oskar mRNAs contain different UTR sequences.

The bicoid UTR causes it to be localized tn the ┬┐interior pole while the distinct oskar UTR sequence causes localization in the posterior plaim An engineered oskar mRNA thijt contains the bicoid UTR is localized to the anterior pole, just tike the normal bicoid mRNA This mislocaliiation of oskar causes the formation ot poie cells in anterior regions Pole cells also form from the posterior pole due to localization of the normal oskar mRNA in the posterior plasm.

The Bicoid Gradient Regulates the Expression of Segmentation Genes in a Concentration-Dependent Eashion

The Bicoid regulatory protein is synthesized prior to the completion of cellularization. As a result, it diffuses away from its source of synthesis at the anterior pole and becomes distributed in a broad concentration gradient along the length of the early embryo. The Bicoid gradient is formed in a way that is different from the Cli and Dorsal gradients. We have already seen that these latter gradients depend on the differential activation of cell surface receptors. By simply diffusing across the syncitial embryo, Bicoid bypasses the need for cell signaling. Once formed, however, it produces multiple thresholds of gene expression, just like the Gli and Dorsal gradients.

There are peak levels of the Bicoid protein in anterior regions, intermediate levels in central regions, and low levels in posterior regions (Figure 18-18]. Different concentrations of the Bicoid protein are required for the regulation of different target genes, just as we have seen for Dorsal. Peak levels of Bicoid are required for the activation of genes in anterior regions that will form head structures; intermediate levels are sufficient for the activation of those genes required for the differentiation of the thorax. We consider the differential regulation of two Bicoid target genes, orthodenticle and hunchback.

Only high concentrations of Bicoid activate the expression of ortho-denticle, which is essential for the differentiation of head structures (Figure 18-t8a). in contrast, both high and intermediate concenira-

Orthodenticle expression

Bicoid protein orthodenticle enhancer-Bico'id binding sites orthodenticle gene

Bicoid gradient

Bicoid gradient

Bicoid protein orthodenticle enhancer-Bico'id binding sites orthodenticle gene

Orthodenticle gradient

Orthodenticle gradient

i_i orthodenticle enhancer-Bcoid binding sites

Was this article helpful?

0 0

Post a comment