A search for cooperative infection of P1-lysogenic Escherichia coli by restricted lambda phage

Abstract

Phage λ · C that has been propagated in Escherichia coli C plates with very low efficiency on E. coli K. The probability that such restricting cells in log-phase or stationary cultures will become phage-yielders rises sharply as the m.o.i. by λ · C increases. The process, which has been termed cooperative infection, is detectable at an m.o.i. of 2 in this system. Both λ · C and λ · K plate with very low efficiency on E. coli C (P1) and on E. coli K (P1). The question of whether these restricting hosts can support cooperative infection has been examined. Neither λ · C nor λ · K could cooperatively infect E. coli C (P1). In E. coli K (P1), λ · K barely overcame the restriction barrier in stationary cultures only, and only at an m.o.i. of 15 and greater. The efficiency was very much lower than that observed in the case of cooperative infection of E. coli K by λ · C. It was concluded that λ · K cannot cooperatively infect E. coli K (P1). In E. coli K (P1), λ · C cooperatively infected only a small fraction of log-phase or stationary populations. It appeared that this fraction was composed of those cells which could support phage growth when singly infected with λ · K. These cells retained the P1-directed mechanism which modifies λ DNA, but probably lost the P1-directed mechanism which restricts λ DNA. The behavior of restricted λ in the P1-lysogens contrasts sharply with their efficient cooperative infection by restricted phage T1 described by other workers. The biochemical data available on the degradation of restricted phage DNA and the properties so far described for isolated restriction enzymes cannot yet explain why cooperative infection takes place in one system but not in another.