Gap genes are expressed in presence of the right combination and amount of transcription factors. But is there any additional mechanism of timing the expression of the gap genes to ensure that they are expressed at the right time?
Context and Detailed Question
Let's look at gap gene expression in Drosophila:
Firstly, bicoid and nanos are expressed and diffused from anterior and posterior ends respectively and form gradients;
Then, hunchback gets expressed, with bicoid acting as promoter and nanos acting as suppressor;
Then, Krüppel gets expressed, with hunchback acting as both promoter and suppressor - meaning that there must be enough hunchback, but not too much.
(or a video: https://www.youtube.com/watch?v=uaedzlrnBGY)
Makes perfect sense, unless you look at it from dynamic, biochemical perspective:
As soon as there's enough bicoid at the anterior region, hunchback immediately starts building up too: I mean, it can't just choose to wait for bicoid to establish its gradient or something - if there's a promoter for it, it should react accordingly.
And so it "chases" the bicoid gradient, in a manner of speaking, and will continue to chase it until it stumbles into nanos protein, which suppresses it.
All this makes sense so far.
Eventually, there'll be too much hunchback at the anterior region, so Krüppel should cease to be produced - but some quantity of it should be expressed there nonetheless. The final gradient should look like this:
The only way to avoid it is if Krüppel kindly waited until the hunchback establishes its final gradient before even attempting to be expressed. But how is this possible? Is there some mechanism that prevents Krüppel from being expressed too early - and if so, then what releases it? Or maybe there's some mechanism of timing, that prevents genes from being expressed too early?
This Krüppel example is just one examples of several, when there must be some timing control in order for genes to be expressed properly.