Gene-specific changes in induced enzyme synthesis in Escherichia coli infected with phage pX174.

Abstract

A few simple experiments involving measurement of induced enzyme synthesis in Escherichia coli cells infected with single-stranded DNA phage φX174 which indirectly suggest a peculiar type of in vivo control of transcription of the genes of the infected cells are described in this chapter. The intracellular replication of this phage, which contains only eight genes and about 5500 nucleotides in its DNA, may be summed up as follows (1). Upon entry into the cell, the single-stranded circular DNA (“plus” strand) of the virus is quickly converted to a double-stranded circular DNA, called “replicative form” or RF, as a result of synthesis of the complementary strand (“minus” strand) mediated by the host enzymes. This parental RF becomes associated with a site on the bacterial membrane; it then replicates semicon- servatively and symmetrically, releasing a number of daughter RF molecules into the cytoplasm in such a manner that the input viral “plus” strand always remains associated with the site, exchanging partners at each replication. RF replication, for which both host and viral functions are necessary, is initiated about 2–3 min after infection (37°C) and continues until about 12 min, when it practically comes to a halt. At this time, an asymmetrical replication of the daughter RF molecules sets in at a very rapid rate. Phage capsid proteins, which have already been made in the cell, are essential for this switchover to asymmetrical DNA replication. During this stage, only the “minus” strand of the RF molecule serves as template for synthesis of a new complementary “plus” strand, expelling thereby the old “plus” strand, which is immediately encapsulated by the capsid proteins to produce a mature phage.