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.