Complementation of M13am9 infections by plasmid-expressed gp9. E. coli K38 bearing the respective plasmid was mixed with LB top agar containing 1 mM IPTG and poured on an agar plate. After solidification, 10 μL drops of serial diluted M13am9 phage suspensions were applied. The bacterial lawn with K38 cells bearing the empty plasmid only allowed the plaque formation of revertant phage (A). K38 cells expressing the wild-type gp9 from the plasmid (B) showed plaque formation at the 105-fold dilution, similar to the suppressor cells K37 (H). When no IPTG was added to the plate plaque formation was reduced (C). Cells expressing the modified gp9 proteins all showed efficient plaque formation. Gp9-T7 (D), gp9-HA (E), gp9-DT7 (F) and gp9-DHA (G) were analysed.