Viruses have evolved multiple mechanisms for altering gene expression in their host to allow expression of genes required for viral proliferation. Bacteriophage T4, which infects E. coli, expresses three classes of genes (early, middle, and late) at different stages of development.

motB is an early gene located in a dispensable region of the T4 genome. Early work demonstrated that a motB deletion, together with deletion of other T4 genes involved in middle gene expression, is lethal and that MotB can be isolated with DNA, host RNA polymerase and two T4 products, MotA and AsiA, that are required for T4 middle promoter activation. We find that motB is conserved among T4-type phages, and contains a KOW domain, known to be involved in binding nucleic acids or in protein-protein interactions.

To determine motB function, we cloned and purified the protein. We find that MotB binds very tightly and nonspecifically to DNA. MotB production is toxic and suppresses cell division. Microscopy indicates significant remodeling of cellular DNA after expression. However, cell division resumes once MotB protein is cleared. These results suggest that MotB binds host DNA and reversibly interferes with an important bacterial process required for cell division.

To investigate the function of MotB during infection, we generated a motB amber mutant. We performed RNA-seq analyses using NapIV non-suppressing cells infected with either the mutant or T4D+, collecting RNA 5-min post-infection. Although the mutant has no detectable growth phenotype, RNA-seq results (analyzed in CLC genomics workbench; fold change ≥ 2; p-value ≤ 0.05), showed impaired expression of several late genes (7, 8, 15, 20, 22, and 27). Early and middle gene expression was unaffected. Since T4 late gene expression requires phage replication, we speculate that MotB improves late gene expression by binding to host DNA, which could compete with phage DNA for the T4 replication proteins.