A phage encoded GapR nucleoid-associated protein binds a bacterial GapR to deactivate it via an inhibitory hetero-oligomer mechanism

Nucleoid associated proteins (NAPs) play key roles in bacterial chromosome organization and cellular processes. Recent work identified GapR as an essential NAP that is ubiquitous in α-proteobacteria. GapR structures reveal that the dimeric form of GapR can tetramerize in the presence of DNA to form a DNA-encasing clamp and data showed that GapR is required for bacterial replication as it recruits topoisomerases to regions of overtwisted DNA. Interestingly, diverse phages that infect α-proteobacteria encode GapR homologs. To gain insight into how phage GapRs may impact their cellular homologs we performed structure/function analyses on the roseobacterial and roseophage RDJLΦ1 GapR proteins. Crystal structures revealed that the RDJLΦ1 GapR harbors a structure different from the bacterial GapR that is incompatible with DNA binding. Cellular localization and fluorescence binding assays confirmed that the RDJLΦ1 phage GapR does not bind DNA. Strikingly, data show that the RDJLΦ1 GapR and bacterial GapR can form hetero-oligomers, suggesting that the RDJLΦ1 GapR may function as a GapR poison. Indeed, assays showed that the heterodimer forms is unable to bind DNA. Collectively these studies indicate a mechanism by which a phage encoded NAP can derail a host NAP to co-opt the cell and possibly favor phage infectious processes.