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Typically, when performing gene-knockout, the experimenters select one gene to remove/replace-with-junk and then see if the prokaryote can still undergo fission. If it continues to reproduce then the gene is labeled as non-essential; if the organism cannot reproduce then the gene is labeled as essential; and some-times (if the organism can reproduce but only for a certain number of generations) the gene is labeled as quasi-essential.

Typically, if gene X is essential, and you knock-out both gene X and some other gene Y then the organism still dies; this is an example of monotonic behavior. However, this doesn't always have to be the case, it could be that gene X is essential only in the presence of gene Y (for instance if the two proteins produced are in a delicate feedback loop). Is there examples when knocking out gene X makes the organism nonviable, but knocking out gene X and Y maintains viability? In the most extreme case, is there an example where both gene X and Y are essential, but if both are knocked-out then the organism is still viable? I am primarily interested in simple prokaryotes (an answer for Mycoplasma genitalium or Escherichia coli would be best) but more complicated organisms are preferred over no answer.

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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 & 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

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