Restriction in the number of infecting lambda phage genomes that can participate in intracellular growth

Abstract

When nonpermissive cells are infected with a constant, low multiplicity of wild-type phage and varying multiplicities of some amber mutants, the proportion of cells infected with wild-type which produce progeny phage decreases sharply as the mutant multiplicity increases. The am − am + ratio of phages among the progeny remains low (2–5) and constant, independent of the am − am + input ratio for a range of 5 to 25. These results may be interpreted to indicate that only a limited number of input phage DNA molecules (1–5) can be used as templates for the synthesis of progeny phage DNA, and that if the wild-type DNA is not used as template in a mixedly infected cell, it cannot contribute enough wild-type product to allow significant growth of the mutant phage. The am − am + progeny ratios approached the input ratios when the defective phage carried mutations in genes N and P, but not in genes A, V, O, and J. These results would be expected, since it has been shown that the products of genes N and P are synthesized in excess of the amounts required for normal phage growth. It is known that λ DNA replication involves at least two stages, “early” replication proceeds via “theta” intermediates, in the manner described for Escherichia coli chromosome, while “late” replication involves concatemeric intermediates which are the precursors of phage DNA. Since it has been shown that as many as 60 input phage DNA molecules can replicate at least once in infected cells, it is possible that restriction to the number of genomes that can be used as templates for progeny phage DNA synthesis takes place only during the second stage of DNA replication.