Isolation and analysis of Escherichia coli mutants that allow increased replication of bacteriophage lambda

Escherichia coli mutants were isolated that supported the growth of a lambda Ots and, in at least one case, a lambda Bts phage at the normally nonpermissive temperature of 39 degrees C. In one such strain, Ots and Bts suppression ability appeared to be a function of the guaB gene. Ots suppression by the mutant guaB strain was prevented if high levels of guanine or xanthine were present in the medium. No other base had any effect on Ots suppression in this strain. Other strains carrying spontaneous mutations resulting in guanine or xanthine auxotrophy (guaA or guaB lesions, respectively) all allowed lambda Ots replication at 39 degrees C; Ots suppression in these strains was also abolished by addition of guanine to the medium. Thus, reduced intracellular guanine levels resulting from guaA or guaB mutations appeared to suppress the inability of lambda Ots and, at least in some cases, Bts bacteriophage to form plaques at 39 degrees C. In burst size experiments, a guaB mutant produced a larger phage yield per infected cell of both lambda Ots and lambda O+ phage at 39 degrees C than did a similar guaB+ strain. It appeared that a lower-than-normal level of guanine (or a guanine derivative) in these cells may permit unusually efficient lambda replication. The fact that O+ and lambda Ots bursts in the guaB mutant were reduced significantly by addition of exogenous guanine to the medium is consistent with this suggestion. Another strain that suppresses the Ots allele has no known auxotrophic requirements, and suppression in this strain was unaffected by addition of guanine to the medium; however, addition of cytidine to the medium specifically eliminated Ots suppression in this strain. The mutation responsible for allowing Ots replication in this strain is unknown.