Iov

Hunchback protein

Synthetic

High-affinity Bicoid-binding sites

Synthetic

Low-affinity Bicoid-binding sites

Reporter-gene product

Reporter-gene product

â–² EXPERIMENTAL FIGURE 15-20 Maternally derived Bicoid controls anterior/posterior expression of the embryonic hunchback (hb) gene. (a-c) Increasing the number of bicoid genes in mother flies changed the Bicoid gradient in the early embryo, leading to a corresponding change in the gradient of Hunchback protein produced from the hunchback gene in the embryo's genome. The hunchback promoter contains three high-affinity and three low-affinity Bicoid-binding sites. Transgenic flies carrying a reporter gene linked to a synthetic promoter containing either four high-affinity sites (d) or four low-affinity sites (e) were prepared. In response to the same Bicoid protein gradient in the embryo, expression of the reporter gene controlled by a promoter carrying high-affinity Bicoid-binding sites extended more posteriorly than did transcription of a reporter gene carrying low-affinity sites. This result indicates that the threshold concentration of Bicoid that activates hunchback transcription depends on the affinity of the Bicoid-binding site. Bicoid regulates other target genes in a similar fashion. [Adapted from D. St. Johnston and C. Nusslein-Volhard, 1992, Cell 68:201.]

terior end where it is produced from the localized mRNA. As a result, a Bicoid protein gradient is established along the an-teroposterior axis of the syncytial embryo. Evidence that the Bicoid protein gradient determines anterior structures was obtained through injection of synthetic bicoid mRNA at different locations in the embryo. This treatment led to the formation of anterior structures at the site of injection, with progressively more posterior structures forming at increasing distances from the injection site. Another test was to make flies that produced extra anterior Bicoid protein; in these flies, the anterior structures expanded to occupy a greater proportion of the embryo.

Bicoid protein promotes transcription of the hunchback (hb) gene from the embryo's genome. Transcription of hunchback is greatest in the anterior of the embryo where the Bicoid concentration is highest. Mutations in hunchback and several other genes in the embryo's genome lead to large gaps in the an-teroposterior pattern of the early embryo; hence these genes are collectively called gap genes. Several types of evidence indicate that Bicoid protein directly regulates transcription of hunchback. For example, increasing the number of copies of the bicoid gene expands the Bicoid and Hunchback (Hb) protein gradients posteriorly in parallel (Figure 15-20a-c). Analysis of the hunchback gene revealed that it contains three low-affinity and three high-affinity binding sites for Bicoid protein. The results of studies with synthetic genes containing either all high-affinity or all low-affinity Bicoid-binding sites demonstrated that the affinity of the site determines the threshold concentration of Bicoid at which gene transcription is activated (Figure 15-20d, e). In addition, the number of Bicoid-binding sites occupied at a given concentration has been shown to determine the amplitude, or level, of the transcription response.

Findings from studies of Bicoid's ability to regulate transcription of the hunchback gene show that variations in the levels of transcription factors, as well as in the number or affinity of specific regulatory sequences controlling different target genes, or both, contribute to generating diverse patterns of gene expression in development. These findings thus parallel those on the Dorsal transcription factor discussed previously. Similar mechanisms are employed in other developing organisms.

Maternally Derived Translation Inhibitors Reinforce Bicoid-Mediated Anterioposterior Patterning

Cell types at the posterior end of the fly embryo are controlled by a different mechanism—one in which control is at the translational level rather than the transcriptional level. As just discussed, transcription of the embryo's hunchback gene, which promotes anterior cell fates, produces an anteriorly located band of hunchback mRNA and Hunchback protein because of the anterior ^ posterior gradient of maternally derived Bicoid protein. In addition, however, hunch-

Lower Your Cholesterol In Just 33 Days

Lower Your Cholesterol In Just 33 Days

Discover secrets, myths, truths, lies and strategies for dealing effectively with cholesterol, now and forever! Uncover techniques, remedies and alternative for lowering your cholesterol quickly and significantly in just ONE MONTH! Find insights into the screenings, meanings and numbers involved in lowering cholesterol and the implications, consideration it has for your lifestyle and future!

Get My Free Ebook


Post a comment