Defective packing of an unusual DNA in a virulent Erwinia phage, Erh 1

Abstract

A newly isolated bacteriophage, Erh 1, for Erwinia herbicola, has been characterized. This virulent phage has been found to have an elongated rod-like head and a short complex tail structure. One major protein and 5 minor proteins have been identified as phage components. The head structure was found to be transparent, flexible and could be twisted or flattened by various treatments. The DNA, isolated from highly purified phage particles, was linear, double-stranded, had a G-C content of 46-47% and displayed two unique features. (1) The isolated phage DNA molecules were highly heterogeneous in contour lengths; the most prevalent molecules had a length of 6-12 mu, but molecules have been measured with lengths ranging from 2.3 mu to 37 mu. (2) One or two long single-stranded regions, "gaps," ranging in length from 0.3 to 3.1 mu with an average length of 1.4 +/- 0.7 mu, were found in about 25% of the phage DNA molecules. Upon density gradient centrifugation of p32 labeled phage, it was found that most of the DNA was contained in apparently noninfectious, defective particles, with densities ranging from 1.34 to 1.41 while most of the infectious particles were found in fractions of density of 1.44 +/- 0.02. When used to multiply infect cells, the lower density particles were able to complement each other and form infectious centers. Further, it was found that infectious particles themselves were heterogeneous and had sedimentation constants varying from 600 S to 1400 S. From the distribution of DNA sizes in these particles, the variations in sedimentation behavior, and the flexibility of the head structure of most particles, it appears that the head structure is formed first and then the DNA is packed inefficiently into this head structure. Apparently, most phage particles are only partly filled and do not contain a complete genome while a few others may contain a large amount of redundant viral DNA.