Induction of lambdoid prophages by amino acid deprivation: differential inducibility; role of recA.

Lambda prophage in auxotrophic lysogens can be induced by omission of one or combinations of the required amino acids from the culture medium. Such amino acid deprivation can result in nearly as effective induction of lambda as thymine deprivation. Prophage 424 is also induced equally effectively under both conditions although to a lesser extent than lambda. By contrast prophage 21 and lambda i21 are differentially induced effectively by thymine deprivation and virtually not at all during amino acid deprivation. The same differential induction of 21 and equivalent induction of lambda and 424 occur when all three prophages are present in the same lysogen. Increasing the levels of lambda repressor with a cI carrying-plasmid prevented amino acidless induction of lambda as did the lambda ind- mutation. A recA, but not a recB, mutation in the host prevented induction by amino acid deprivation. A recC mutant host showed increased spontaneous induction of lambda and 21 prophages. The findings reported are used as an argument that the recA protease probably is not itself acting as the inducing protease and that a likely source of the observed specificity is an effector molecule. Different effector molecules may be produced in response to different exigent situations, to which the phage repressors may have evolved sensitivity. lambda i80 was inducible both by amino acid and thymine deprivation.

Effect of cI repressor level on thymineless and spontaneous induction; specificity of lambda RNA transcription.

To determine the role of the cI repressor in induction provoked by thymine deprivation, we have analyzed lambda messenger RNA made during and the effect of cI repressor levels on thymineless induction. During thymineless induction, the l- and r-strand transcription of lambda is restricted to the "early" and "delayed early" RNA. This transcriptional pattern is similar to that reported for lambda mutants defective in DNA synthesis. "Late" r-strand transcription requires the addition of thymine. A decrease (to less than 10% of 0 time) in the amount of exogenous label (3H-uridine) incorporated into total RNA by the time of maximum thymineless induction was observed. Since subsequent burst sizes are not diminished by the thymine deprivation and competition experiments show that the amount of lambda message RNA present is at least as great as that in heat induced lambda cI857 lysogens, this decrease must involve either enlarged uridine pool sizes or decreased entry of label. The introduction into the lambda lysogen of a plasmid (pKB252) carrying the lambda cI gene prevents (1) the thymineless induction of lambda (curing the plasmid restores thymineless induction) and, (2) the appearance of both spontaneously induced cells and free phage. Thus, thymineless induction is dependent on the level of cI repressor and spontaneous induction also appears to be the consequence of lowered repressor levels in lambda lysogens.

Precursor and product in bacteriophage lambda recombination.

Analyses of the time of recombination in bacteria infected with phage lambda and in spheroplasts infected with purified phage lambda DNA indicate a preponderance of recombination occurring before replication ("early recombination") after infection with tandem dimers formed in vitro compared to linear monomer DNA and phage infections. Among the early recombinants is a large fraction of cells that produces one recombinant type exclusively. Two suggestions are discussed-that a dimer-like molecule is the precursor of the recombinant progeny molecule and that one recombinant chromosome is the frequent or exclusive product of a recombinational event.

Requirements for macromolecular synthesis in the establishment of beta-galactosidase repression in zygotes.

Inhibitors of protein synthesis do not consistently prevent formation of the lac operon repressor, according to several published reports, although direct evidence indicates that the repressor is a protein. Inhibition of ribonucleic acid (RNA) synthesis has never been shown to block lactose repression. These results have raised the possibility that repressor is synthesized in some unusual fashion. We have studied the effect of various inhibitors upon the establishment of repression in zygotes, utilizing conditions which minimize catabolite repression. Inhibition of protein synthesis by either chloramphenicol treatment or tryptophan deprivation blocked repressor formation in our experiments. Sodium borate and 6-azauracil are compounds reported to be specific inhibitors of RNA synthesis, and their behavior in control experiments is consistent with this specificity. Both delayed the establishment of repression. Thymine deprivation, either by starvation of a thymine auxotroph or by treatment with 5-fluorodeoxyuridine, did not delay the onset of repression. We conclude that repressor formation requires RNA synthesis and probably utilizes the usual protein-forming mechanisms.