Two distinct SSB protein families in nucleo-cytoplasmic large DNA viruses

Motivation: Eukaryote-infecting nucleo-cytoplasmic large DNA viruses (NCLDVs) feature some of the largest genomes in the viral world. These viruses typically do not strongly depend on the host DNA replication systems. In line with this observation, a number of essential DNA replication proteins such as DNA polymerases, pri-mases, helicases and ligases have been identified in the NCLDVs. One other ubiquitous component of DNA replisomes is the single-stranded DNA-binding (SSB) protein. Intriguingly, no NCLDV ho-mologs of canonical OB-fold containing SSB proteins had previously been detected. Only in poxviruses, one out of seven NCLDV fami-lies, I3 was identified as the SSB protein. However, whether I3 is related to any known protein structure has not yet been established. Results: Here, we addressed the case of "missing" canonical SSB proteins in the NCLDVs and also probed evolutionary origins of the I3 family. Using advanced computational methods, in four NCLDV families we detected homologs of the bacteriophage T7 SSB protein (gp2.5). We found the properties of these homologs to be consistent with the SSB function. Moreover, we implicated specific residues in ssDNA binding. At the same time, we found no evolutionary link between the T7 gp2.5-like NCLDV SSB homologs and the poxviral SSB protein (I3). Instead, we identified a distant relationship between I3 and the bacterial RNA-binding protein SmpB. Thus, apparently, the NCLDVs have the two major distinct sets of SSB proteins having bacteriophage and bacterial origins respectively.