Origin of the Gradients

BCD

-minus end of microtubules at anterior pole

-maternal bcd mRNA tethered to this end

-translation of the BCD protein begins and it diffuses in common cytoplasm

-becomes localized in nuclei and the nuclei closest to the source of translation (A-pole) get the highest concentration of BCD protein.

-result is the steep BCD gradient

HB-M

-produced by means of pole-transcriptional regulation of hb-m mRNA

-translation blocked by NOS protein product (translational repressor protein) encoded by nanos (nos) gene

-nos mRNA localized at the posterior pole because it is associated with the positive end of microtubules

-translation of nos mRNA begins and NOS protein distributed in a gradient opposite that of BCD.

-NOS protein has the ability to inhibit the translation of hb-m mRNA

-the concentration gradient of the NOS translation repress-or produces the shallow gradient of the HB-M protein.

How do the maternal bcd and nos mRNAs get tethered to opposite ends of the polarized microtubules of the oocyte in early syncitial embryo?

-There are specific microtubule-association sequences located within the 3'UTRs (untranslated regions)

-bcd mRNAs 3'UTR localization sequences are bound to a protein which also binds to the negative ends of microtubules

-nos mRNAs 3' UTR localization sequences are bound to molecules in the polar granules which are anchored to the + end of the microtubules.

How do we know that the localization sequences reside on the 3'UTRs of the mRNAs?

-determined by “swapping” experiments.

-synthetic transgene produces mRNA with 5'UTR and protein-coding regions of the normal nos mRNA glued to the 3'UTR of the normal bcd mRNA

-inserted into fly genome

-the nos-bcd mRNA will be localized at anterior end of oocyte.

-results in a double gradient of NOS : one from anterior to posterior, due to the transgene’s mRNA and one from posterior to anterior, due to the normal nos gene’s mRNA

-produces embryo with two mirror-image posteriors and no anterior

-the NOS protein now present throughout the entire embryo translationally represses hb-m mRNA (and bcd mRNA)