MORPHOLOGY AND HYDROLYTIC ACTIVITY OF A7, A TYPING PHAGE OF PSEUDOMONAS-SYRINGAE PV MORSPRUNORUM

Bacteriophage A7 has been employed as an indicator for strains of Pseu domonas syringae pv. morsprunorum race 1. Electron microscopy showed t hat this phage had a hexagonal head, 59 nm in diameter, and a long, fl exible, non-contractile tail, 164 nm long and 8 nm wide, containing ap proximately 15 evenly spaced transverse striations and terminating in a base-plate equipped with six spikes arranged in a radially symmetric al pattern around a central core. Two short fibres projecting at an an gle from the base-plate were also visible on some phage particles. Pha ge A7 can therefore be placed in group B of the classification system of Bradley. Phage particles bound at apparently random sites over the surface of host cells by their base-plates, and after a short time rel eased DNA from their heads. Phage A7 uses lipopolysaccharide (LPS) as its binding site on the bacterial cell surface, removing the D-rhamnan side-chains by the action of a rhamnan hydrolase. The appearance of p urified LPS by electron microscopy was either strand-like or vesicular , according to whether it had been stained with phosphotungstate or ur anyl acetate respectively. Strands were of approximately uniform width (approx. 9 nm). Vesicular forms included both circles, 25-45 nm in di ameter, and larger oval structures of greater electron transparency. T he appearance of the LPS did not alter on addition of the phage. Phage particles were observed attached via their base-plates to LPS vesicle s, in particular the larger oval vesicles, but were never seen attache d to strands. The heads of phage particles attached to LPS were freque ntly empty. The rhamnanase of phage A7 released the side-chain residue s from the LPS as an equimolar mixture of tri- and hexasaccharide. By using H-1-NMR spectroscopy and methylation analysis, the site of cleav age has been identified as the 2)-alpha-D-Rhap(1 --> 3) residue.