Introduction:ϕX—Some Recollections

Abstract

The origins of the research in my laboratory on the bacteriophage ϕX174 have been recounted in the chapter “ϕX: Multum in Parvo” in Phage and the Origins of Molecular Biology (Sinsheimer 1966). The more salient events in the particular research program (in collaboration with Dr. Arthur Kornberg’s laboratory) which led to the first in vitro synthesis of an infective DNA are detailed in the article “Closing the Ring” (Sinsheimer 1968).

ϕX Rings
One of the most significant and unanticipated discoveries concerning ϕX was the circularity of its DNA molecule. This first demonstration of a ring form of DNA and the accompanying development of criteria for the verification of such ring structures led quite directly to their demonstration in other viral forms (polyoma, SV40, lambda DNA during the eclipse phase, etc.) and elsewhere (in plasmids, ribosomal DNA, etc.).

The concept that ϕX DNA might be a circular molecule arose from the negative results of persistent experiments by Dr. Walter Fiers to identify the nucleotides at the putative ends of ϕX DNA (Fiers and Sinsheimer 1962a). ϕX DNA was the first relatively homogeneous DNA preparation available with a known size (about 5500 nucleotides) and an assayable biological activity; therefore, it seemed reasonable to examine its termini for any unusual features or regularities.

Dr. Fiers attempted to identify the assumed terminal nucleotides by controlled exonuclease digestion from both the 3′ and 5′ ends. When the 3′ or 5′ exonuclease preparations used were effectively devoid of endonuclease activity, only a very small release of...