Elements in the λ immunity region regulate phage development: beyond the 'Genetic Switch'

The crystal structure of bacteriophage λ RexA provides novel insights into the DNA binding properties of Rex-like phage exclusion proteins

RexA and RexB function as an exclusion system that prevents bacteriophage T4rII mutants from growing on Escherichia coli λ phage lysogens. Recent data established that RexA is a non-specific DNA binding protein that can act independently of RexB to bias the λ bistable switch toward the lytic state, preventing conversion back to lysogeny. The molecular interactions underlying these activities are unknown, owing in part to a dearth of structural information. Here, we present the 2.05-Å crystal structure of the λ RexA dimer, which reveals a two-domain architecture with unexpected structural homology to the recombination-associated protein RdgC. Modelling suggests that our structure adopts a closed conformation and would require significant domain rearrangements to facilitate DNA binding. Mutagenesis coupled with electromobility shift assays, limited proteolysis, and double electron-electron spin resonance spectroscopy support a DNA-dependent conformational change. In vivo phenotypes of RexA mutants suggest that DNA binding is not a strict requirement for phage exclusion but may directly contribute to modulation of the bistable switch. We further demonstrate that RexA homologs from other temperate phages also dimerize and bind DNA in vitro. Collectively, these findings advance our mechanistic understanding of Rex functions and provide new evolutionary insights into different aspects of phage biology.

Insights into Superinfection Immunity Regulation of Xanthomonas axonopodis Filamentous Bacteriophage cf

Filamentous bacteriophage cf infects Xanthomonas axonopodis pv. citri, a serious plant pathogen which causes citrus canker. To understand the immunity regulation of bacteria infected with bacteriophage cf, we applied DNA shuffling to mutate the cf intergenic region. One of the immunity mutants, cf-m3 (NCBI Taxonomy ID: 3050368) expressed a 106-109 fold greater superinfection ability compared with wild type cf. Nine mutations were identified on the cf-m3 phage, four of which were located within the coding region of an open reading frame (ORF165) for a hypothetical repressor, PT, and five located upstream of the PT coding region. A set of phages with mutations to the predicted PT protein or the upstream coding region were generated. All showed similarly low superinfection efficiency to wild type cf and no superinfection ability on cf lysogens. The results indicate that rather than superinfection inhibition, the PT protein and the un-transcribed cis element function individually as positive regulators of cf superinfection immunity. Greater superinfection ability depends on the simultaneous presence of both elements. This work yields further insight into the possible control of citrus canker disease through phages that overcome host superinfection immunity.

Recombineering: Genetic Engineering in Escherichia coli Using Homologous Recombination

The bacterial chromosome and bacterial plasmids can be engineered in vivo by homologous recombination using either PCR products or synthetic double-stranded DNA (dsDNA) or single-stranded DNA as substrates. Multiple linear dsDNA molecules can be assembled into an intact plasmid. The technology of recombineering is possible because bacteriophage-encoded recombination proteins efficiently recombine sequences with homologies as short as 35 to 50 bases. Recombineering allows DNA sequences to be inserted or deleted without regard to the location of restriction sites and can also be used in combination with CRISPR/Cas targeting systems. © 2023 Wiley Periodicals LLC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA. Basic Protocol: Making electrocompetent cells and transforming with linear DNA Support Protocol 1: Selection/counter-selections for genome engineering Support Protocol 2: Creating and screening for oligo recombinants by PCR Support Protocol 3: Other methods of screening for unselected recombinants Support Protocol 4: Curing recombineering plasmids containing a temperature-sensitive replication function Support Protocol 5: Removal of the prophage by recombineering Alternate Protocol 1: Using CRISPR/Cas9 as a counter-selection following recombineering Alternate Protocol 2: Assembly of linear dsDNA fragments into functional plasmids Alternate Protocol 3: Retrieval of alleles onto a plasmid by gap repair Alternate Protocol 4: Modifying multicopy plasmids with recombineering Support Protocol 6: Screening for unselected plasmid recombinants Alternate Protocol 5: Recombineering with an intact λ prophage Alternate Protocol 6: Targeting an infecting λ phage with the defective prophage strains.

Study of Ren, RexA, and RexB Functions Provides Insight Into the Complex Interaction Between Bacteriophage λ and Its Host, Escherichia coli

The phage λ rexA and rexB genes are expressed from the P RM promoter in λ lysogens along with the cI repressor gene. RexB is also expressed from a second promoter, P LIT, embedded in rexA. The combined expression of rexA and rexB causes Escherichia coli to be more ultraviolet (UV) sensitive. Sensitivity is further increased when RexB levels are reduced by a defect in the P LIT promoter, thus the degree of sensitivity can be modulated by the ratio of RexA/RexB. Expression of the phage λ ren gene rescues this host UV sensitive phenotype; Ren also rescues an aberrant lysis phenotype caused by RexA and RexB. We screened an E. coli two-hybrid library to identify bacterial proteins with which each of these phage proteins physically interact. The results extend previous observations concerning λ Rex exclusion and show the importance of E. coli electron transport and sulfur assimilation pathways for the phage.

Bacteriophage λ RexA and RexB Functions Assist the Transition from Lysogeny to Lytic Growth

The CI and Cro repressors of bacteriophage λ create a bistable switch between lysogenic and lytic growth. In λ lysogens, CI repressor expressed from the P_RM promoter blocks expression of the lytic promoters P_L and P_R to allow stable maintenance of the lysogenic state. When lysogens are induced, CI repressor is inactivated and Cro repressor is expressed from the lytic P_R promoter. Cro repressor blocks P_RM transcription and CI repressor synthesis to ensure that the lytic state proceeds. RexA and RexB proteins, like CI, are expressed from the P_RM promoter in λ lysogens; RexB is also expressed from a second promoter, P_LIT , embedded in rexA. Here we show that RexA binds CI repressor and assists the transition from lysogenic to lytic growth, using both intact lysogens and defective prophages with reporter genes under control of the lytic P_L and P_R promoters. Once lytic growth begins, if the bistable switch does return to the immune state, RexA expression lessens the probability that it will remain there, thus stabilizing the lytic state and activation of the lytic P_L  and P_R  promoters. RexB modulates the effect of RexA and may also help establish phage DNA replication as lytic growth ensues.