Mimivirus encodes an essential MC1-like non-histone architectural protein involved in DNA condensation

The first giant virus discovered, Acanthamoeba polyphaga mimivirus (APMV), has a 1.2 Mb dsDNA genome organized as genomic fiber within the capsid. This fiber is comprised of a proteinaceous shell of ∼30 nm diameter that encloses the folded DNA. Surprisingly, for the assembly of the enormous genome of APMV, no DNA condensing protein has been reported to date. Our analysis of the uncharacterized packaged protein complement of Mimivirus led to the identification of a putative DNA-bending archaeal MC1 domain in a hypothetical protein (gp275) coded by the R252 gene. Gene knock-out analysis shows that gp275 is critical for viral multiplication. Biochemical and microscopic characterization further demonstrates the compaction of DNA upon binding to gp275. Together, this study suggests that gp275 is an MC1-like architectural protein involved in the organization of the genomic DNA within the capsid of Mimivirus.