synthetic viable

Mutation A - lethal
Mutation B - lethal
Mutation A + B - Viable
Is anyone aware of the above situation which seems to be the opposite of “synthetic lethal” ?

I imagine there are many where each mutation causes lethality for the opposite reason. One is too much, the other is too little. The double might be ‘just right.’

An example would be in the FGF tyrosine kinase signaling pathway. egl-15 null mutants are lethal (scrawny and sterile). Null mutations in clr-1, a phosphatase which opposes egl-15, are also lethal, but overfilled with fluid, the ‘opposite’ of scrawny. Double mutants are called suppressors of clear, or SOC. They are viable, and depending on the combination of the two mutations, can look wild type.


Two other examples are 1) the neddylation E1-activating enzyme rfl-1 and components of the COP-9 signalosome, and 2) the dynein heavy chain dhc-1 and the light chain dyrb-1.

You can probably find additional examples by searching the literature for suppressor screens of lethal mutations; in some cases, the suppressors are lethal as single mutants.