Sinorhizobium meliloti Phage M9 Defines a New Group of T4 Superfamily Phages with Unusual Genomic Features but a Common

Relatively little is known about the phages that infect agriculturally important nitrogen-fixing rhizobial bacteria. Here we report the genome and cryo-electron microscopy structure of the Sinorhizobiummeliloti-infecting T4 superfamily phageM9. This phage and its close relative Rhizobium phage vB_RleM_P10VF define a new group of T4 superfamily phages. These phages are distinctly different from the recently characterized cyanophage-like S. meliloti phages of theM12 group. Structurally,M9 has a T16 capsid formed from repeating units of an extended gp23-like subunit that assemble through interactions between one subunit and the adjacent E-loop insertion domain. Though genetically very distant from the cyanophages, theM9 capsid closely resembles that of the T4 superfamily cyanophage Syn9.M9 also has the same T16 capsid architecture as the very dis-tant phage SPO1 and the herpesviruses. Despite their overall lack of similarity at the genomic and structural levels,M9 and S. meliloti phageM12 have a small number of open reading frames in common that appear to encode structural proteins involved in interaction with the host and which may have been acquired by horizontal transfer. These proteins are predicted to encode tail baseplate proteins, tail fibers, tail fiber assembly proteins, and glycanases that cleave host exopolysaccharide. IMPORTANCE Despite recent advances in the phylogenetic and structural characterization of bacteriophages, only a small number of phages of plant-symbiotic nitrogen-fixing soil bacteria have been studied at the molecular level. The effects of phage predation upon bene