Non-monotonic knock-out effects in prokaryotes
Here is an example in which the knockout of a non-essential gene in yeast (Saccharomyces cerevisiae) suppresses the lethal effects of a different knockout.


  Amy L. Kistler and Christine Guthrie (2001) Deletion of MUD2, the yeast homolog of U2AF65, can bypass the requirement for Sub2, an essential spliceosomal ATPase. Genes &amp; Development 15:42-49


The SUB2 gene encodes a protein which is a component of the TREX complex required for nuclear mRNA export. Deletion of the SUB2 gene is lethal. The MUD2 gene encodes a non-essential protein involved in pre-mRNA splicing.

The authors show that - 


  "...the essential requirement for the SUB2 ATPase can be bypassed by deletion of the nonessential splicing factor MUD2..."


Detailed information about these (and all other) Saccharomyces cerevisiae genes is freely available at the Saccharomyces Genome Database