Overview: A Comparison of the Isometric and Filamentous Phages

Abstract

This overview is written partly as a summary and partly with the intent to make a comparison between the isometric and filamentous phages. I hope in this way to cover most of the salient points made in this book which might be of interest to the casual reader. Because these two phage types are structurally so patently dissimilar, it might seem unlikely that they are related; nevertheless, underlying similarities have led me to argue that there is a close evolutionary connection (Denhardt 1975). Inevitably, however, the argument is weakened by the fact that similarities do not necessarily mean that a close evolutionary relationship exists—they may reflect convergent evolution or they may be coincidental. It is hoped that as we learn more about these viruses the truth will become clear. I have excluded the mycoplasma viruses (Maniloff et al.)1 from this discussion because not enough is known about them to allow a meaningful comparison with the coliphages.

The Virion
The articles by Day and Wiseman, by Marvin, and by Makowski and Caspar detail what we know about the structure of the filamentous phage virion—not only of the coliphages, but also of the Pseudomonas phage Pf1 and the Xanthomonas phage Xf. General agreement on the details of the structure of the f1-fd-M13 group of phages is lacking, and despite the similarity of their X-ray diffraction patterns, the Pf1 and Xf virions seem to be constructed on the basis of different protein-DNA interactions. In all of these viruses, circular, single-stranded (SS)...