Natural examples of XOR functions at the cellular level
When I think about your question of natural examples of XOR, it pushes me to think about what type of natural environments (i.e., evolutionary pressures) would lead to the selection of an XOR equivalent.  

When we implemented a synthetic XOR by "double flipping" one transcription terminator as a type of gene expression "check valve" it was the case that I did not know of any natural examples of flipping terminators, whereas I know of several examples of nature flipping promoters.  

Why do we have plenty of natural examples of flipping promoters but (unknown to me at least) as yet no charismatic examples of naturally flipping terminators?  

Perhaps we haven't been looking in the right environments to find them?

E.g., for a terminator to exist in nature, there needs to be something that must  be expressed upstream of the terminator, and also something that should not be expressed downstream of the terminator.  Since there are plenty of terminators there must plenty of examples of nature selecting for terminators.  My favorite example is the "Tø" terminator in phage T7 that is just downstream of the major capsid gene (highly expressed) but just upstream of a minor particle protein (weakly expressed).

But now reason thru what would need to be true in order to select for terminator flipping.  I'd need all the requirements of a static terminator (above), and then another such pairing of selective requirements under different environmental circumstances for additional gene products encoded in the opposite direction on the complementary strand of DNA. Phew! 

I'm not saying that it is impossible to imagine such a complicated coupling of selection requirements emerging in nature, but it definitely seems a much more improbable aggregation of selective pressures to co-mingle all within a single genetic loci.  

I'm still nevertheless very curious and would be grateful to learn of any natural examples.  -DE