Question

1. One of the main events in establishing the dorsal/ventral axis of Drosophila embryos involves the transcription factor dorsal. As you remember from lecture, dorsal is a transcription factor that activates transcription of the twist and rhomboid genes.

• dorsal’s activity can be inhibited by a protein called cactus. (Cactus does this by binding to dorsal and not letting it enter the nucleus.) Spaetzle, an extracellular signal, inhibits cactus, (after Spaetzle binds to Spaetzle’s receptor), releasing dorsal. Review your lecture slides to refresh your memory, then answer the following questions:

1. (4) Using the symbols: “à” to indicate a positive effect and “––|” (a blunt-ended arrow) to indicate a negative effect, to draw out the regulatory pathway summarized above. In other words, what are the steps by which spätzle stimulates transcription of rhomboid? Single words (the name of the protein or gene) are sufficient for each step (3 points).
2. (2) Give two examples of mutations that would result in no rhomboid transcription, even in the presence of spätzle and briefly explain (3 points).
3. (2) Now, give two examples of mutations that would result in transcription of rhomboid even in the absence of spätzle and briefly explain (3 points).

Answer 1

Ans; a) The regulatory pathway of the transcription of rhomboid genes takes place is drawn in the picture enclosed below:

b) As given in the question that Spaetzle, an extracellular signal inhibits cactus after binding to Spaetzle's receptor and thus enhancing the rhomboid genes transcription. Now if because of the mutation Spaetzle is not able to bind to its receptor then it is not being able to inhibit Cactus and hence no transcription of rhomboid genes takes place even in the presence of the Spaetzle  Now two possible mutations that would result in the latter will be i) Non-sense mutation in Spaetzle receptor gene: A nonsense mutation is a genetic mutation in a DNA sequence that results in a shorter, unfinished protein product. So non sense mutation will produce a truncated receptor to which Spaetzle will not be able to bind and hence not able to function. ii) Non-sense mutation in Spaetzle gene: As spaetzle is also a protein, non-sense mutation in its gene will cause a truncated protein which will be not able to generate desired impact within the cell.

c)  As given in the question that Spaetzle, an extracellular signal inhibits cactus after binding to Spaetzle's receptor and thus enhancing the rhomboid genes transcription. Now if spaetzle is absent then cactus will not be inhibited by spaetzle hence cactus will be able to inhibits the dorsal transcription factor and no transcription of rhomboid genes takes place.Now Cactus inhibits dorsal by binding to dorsal and not letting it enter the nucleus so if the mutation is able to disrupt the binding site of cactus then transcription of rhomboid genes will takes place even in the absence of spaetzle. Now examples of such type of mutation are: i) Non-sense mutation in cactus gene: As i described it above this mutation will result a truncated protein which is not being able of performing its task and hence it will not being able to inhibit dorsal and transcription of rhomboid genes still takes place. ii) Missense Mutation in cactus gene: It is also a single base substitution and in this mutation base substitution results in the generation of a codon that specifies a different amino acid and hence leads to a different polypeptide sequence. And because of this change the structure and properties of the protein of interest will be changed and its activity dimnished and hence cactus will not being able of inhibiting dorsal and so the transcription of rhomboid genes will be continued in the absense of spaetzle.