The **py**rimidine **c**yclase **s**ystem for **a**ntiphage **r**esistance (Pycsar) :ref{doi=10.1016/j.cell.2021.09.031} is composed of at least two proteins: a cyclase and diverse effectors. The Pycsar defense system is related to the [CBASS](/defense-systems/cbass) but differs in the signaling molecule produced by the cyclase.
In turn, phages encode anti-Pycsar (Apyc) proteins encoded that counteract Pycsar defenses :ref{doi=10.1038/s41586-022-04716-y}.
## Molecular mechanisms
Pycsar uses cyclic nucleotide (pyrimidine cyclase, cCMP and cUMP) signals to induce cell death via diverse effectors activation and prevent viral propagation.
While Apyc degrades cyclic nucleotide signals to prevent host immune responses :ref{doi=10.1016/j.cell.2021.09.031,10.1038/s41586-022-04716-y}.
## Example of genomic structure
## Example of genomic structure
The Pycsar is composed of at least 2 proteins: a Cyclase and an Effector.
The Pycsar is composed of at least 2 proteins: a Cyclase and an Effector.
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@@ -40,6 +57,19 @@ Proportion of genome encoding the Pycsar system for the 14 phyla with more than
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@@ -40,6 +57,19 @@ Proportion of genome encoding the Pycsar system for the 14 phyla with more than
## Structure
## Structure
### Experimentaly determined structure
From :ref{doi=10.1016/j.cell.2021.09.031} in *Burkholderia cepacia* LK29: