ABSTRACT
CI represses cro; Cro represses cI. This double negative feedback loop is the core of the classical CI-Cro epigenetic switch of bacteriophage lambda. Despite the classical status of this switch, the role in lambda development of Cro repression of the P(RM) promoter for CI has remained unclear. To address this, we created binding site mutations that strongly impaired Cro repression of P(RM) with only minimal effects on CI regulation of P(RM). These mutations had little impact on lambda development after infection but strongly inhibited the transition from lysogeny to the lytic pathway. We demonstrate that following inactivation of CI by ultraviolet treatment of lysogens, repression of P(RM) by Cro is needed to prevent synthesis of new CI that would otherwise significantly impede lytic development. Thus a bistable CI-Cro circuit reinforces the commitment to a developmental transition.
Subject(s)
Bacteriophage lambda/growth & development , DNA-Binding Proteins/metabolism , Gene Expression Regulation, Viral , Lysogeny/genetics , Repressor Proteins/metabolism , Viral Regulatory and Accessory Proteins/metabolism , Virus Activation/genetics , Bacteriophage lambda/genetics , Base Sequence , DNA-Binding Proteins/antagonists & inhibitors , DNA-Binding Proteins/genetics , DNA-Binding Proteins/radiation effects , Molecular Sequence Data , Mutation , Promoter Regions, Genetic/genetics , Prophages/genetics , Prophages/physiology , Repressor Proteins/antagonists & inhibitors , Repressor Proteins/genetics , Repressor Proteins/radiation effects , Viral Regulatory and Accessory Proteins/antagonists & inhibitors , Viral Regulatory and Accessory Proteins/genetics , Viral Regulatory and Accessory Proteins/radiation effectsABSTRACT
PURPOSE: To determine whether the quality of ionizing radiation is critical for activation of a radiation-specific DNA binding protein. METHODS AND MATERIALS: We have previously shown that after exposing Epstein Barr virus-transformed lymphoblastoid cells to ionizing radiation, a specific DNA binding factor appears in the nucleus apparently as a result of translocation from the cytoplasm. This protein binds to a number of different genomic sequences and a consensus motif has been identified. Because the protein was not activated by UV light, it was of interest whether high linear energy transfer (LET) radiation was capable of activation. RESULTS: We describe here the activation of a specific DNA binding protein by high LET neutron radiation. The protein binds a region adjacent to and overlapping with the distal repeat within a 179 base-pair fragment of the well-characterized Simian Virus (SV40) bidirectional promoter/enhancer element. The appearance of the DNA binding activity was dose dependent and reached a maximum level by 90 min postirradiation. A reduction in DNA binding activity was evident at later times after irradiation. CONCLUSIONS: The specific nature of this response and the rapidity of activation may indicate a pivotal role for this protein in repair or in some other aspect of the cellular response to radiation damage.
