Short-range and long-range context effects on coliphage T4 endonuclease II-dependent restriction

Structure of bacteriophage T4 endonuclease II mutant E118A, a tetrameric GIY-YIG enzyme

Coliphage T4 endonuclease II (EndoII), encoded by gene denA, is a small (16 kDa, 136 aa) enzyme belonging to the GIY-YIG family of endonucleases, which lacks a C-terminal domain corresponding to that providing most of the binding energy in the structurally characterized GIY-YIG endonucleases, I-TevI and UvrC. In vivo, it is involved in degradation of host DNA, permitting scavenging of host-derived nucleotides for phage DNA synthesis. EndoII primarily catalyzes single-stranded nicking of DNA; 5- to 10-fold less frequently double-stranded breaks are produced. The Glu118Ala mutant of EndoII was crystallized in space group P2(1) with four monomers in the asymmetric unit. The fold of the EndoII monomer is similar to that of the catalytic domains of UvrC and I-TevI. In contrast to these enzymes, EndoII forms a striking X-shaped tetrameric structure composed as a dimer of dimers, with a protruding hairpin domain not present in UvrC or I-TevI providing most of the dimerization and tetramerization interfaces. A bound phosphate ion in one of the four active sites of EndoII likely mimics the scissile phosphate in a true substrate complex. In silico docking experiments showed that a protruding loop containing a nuclease-associated modular domain 3 element is likely to be involved in substrate binding, as well as residues forming a separate nucleic acid binding surface adjacent to the active site. The positioning of these sites within the EndoII primary dimer suggests that the substrate would bind to a primary EndoII dimer diagonally over the active sites, requiring significant distortion of the enzyme or the substrate DNA, or both, for simultaneous nicking of both DNA strands. The scarcity of potential nucleic acid binding residues between the active sites indicates that EndoII may bind its substrate inefficiently across the two sites in the dimer, offering a plausible explanation for the catalytic preponderance of single-strand nicks. Mutations analyzed in earlier functional studies are discussed in their structural context.

Bacteriophage T4 endonuclease II, a promiscuous GIY-YIG nuclease, binds as a tetramer to two DNA substrates

The oligomerization state and mode of binding to DNA of the GIY-YIG endonuclease II (EndoII) from bacteriophage T4 was studied using gel filtration and electrophoretic mobility shift assays with a set of mutants previously found to have altered enzyme activity. At low enzyme/DNA ratios all mutants except one bound to DNA only as tetramers to two DNA substrates. The putatively catalytic E118 residue actually interfered with DNA binding (possibly due to steric hindrance or repulsion between the glutamate side chain and DNA), as shown by the ability of E118A to bind stably also as monomer or dimer to a single substrate. The tetrameric structure of EndoII in the DNA-protein complex is surprising considering the asymmetry of the recognized sequence and the predominantly single-stranded nicking. Combining the results obtained here with those from our previous in vivo studies and the recently obtained crystal structure of EndoII E118A, we suggest a model where EndoII translocates DNA between two adjacent binding sites and either nicks one strand of one or both substrates bound by the tetramer, or nicks both strands of one substrate. Thus, only one or two of the four active sites in the tetramer is catalytically active at any time.

Amino acid residues in the GIY-YIG endonuclease II of phage T4 affecting sequence recognition and binding as well as catalysis

Phage T4 endonuclease II (EndoII), a GIY-YIG endonuclease lacking a carboxy-terminal DNA-binding domain, was subjected to site-directed mutagenesis to investigate roles of individual amino acids in substrate recognition, binding, and catalysis. The structure of EndoII was modeled on that of UvrC. We found catalytic roles for residues in the putative catalytic surface (G49, R57, E118, and N130) similar to those described for I-TevI and UvrC; in addition, these residues were found to be important for substrate recognition and binding. The conserved glycine (G49) and arginine (R57) were essential for normal sequence recognition. Our results are in agreement with a role for these residues in forming the DNA-binding surface and exposing the substrate scissile bond at the active site. The conserved asparagine (N130) and an adjacent proline (P127) likely contribute to positioning the catalytic domain correctly. Enzymes in the EndoII subfamily of GIY-YIG endonucleases share a strongly conserved middle region (MR, residues 72 to 93, likely helical and possibly substituting for heterologous helices in I-TevI and UvrC) and a less strongly conserved N-terminal region (residues 12 to 24). Most of the conserved residues in these two regions appeared to contribute to binding strength without affecting the mode of substrate binding at the catalytic surface. EndoII K76, part of a conserved NUMOD3 DNA-binding motif of homing endonucleases found to overlap the MR, affected both sequence recognition and catalysis, suggesting a more direct involvement in positioning the substrate. Our data thus suggest roles for the MR and residues conserved in GIY-YIG enzymes in recognizing and binding the substrate.

Biochemical analysis of the substrate specificity and sequence preference of endonuclease IV from bacteriophage T4, a dC-specific endonuclease implicated in restriction of dC-substituted T4 DNA synthesis

Endonuclease IV encoded by denB of bacteriophage T4 is implicated in restriction of deoxycytidine (dC)-containing DNA in the host Escherichia coli. The enzyme was synthesized with the use of a wheat germ cell-free protein synthesis system, given a lethal effect of its expression in E.coli cells, and was purified to homogeneity. The purified enzyme showed high activity with single-stranded (ss) DNA and denatured dC-substituted T4 genomic double-stranded (ds) DNA but exhibited no activity with dsDNA, ssRNA or denatured T4 genomic dsDNA containing glucosylated deoxyhydroxymethylcytidine. Characterization of Endo IV activity revealed that the enzyme catalyzed specific endonucleolytic cleavage of the 5' phosphodiester bond of dC in ssDNA with an efficiency markedly dependent on the surrounding nucleotide sequence. The enzyme preferentially targeted 5'-dTdCdA-3' but tolerated various combinations of individual nucleotides flanking this trinucleotide sequence. These results suggest that Endo IV preferentially recognizes short nucleotide sequences containing 5'-dTdCdA-3', which likely accounts for the limited digestion of ssDNA by the enzyme and may be responsible in part for the indispensability of a deficiency in denB for stable synthesis of dC-substituted T4 genomic DNA.

Bacteriophage T4 endonuclease II: concerted single-strand nicks yield double-strand cleavage

In vivo, endonuclease II (EndoII) of coliphage T4 cleaves sites with conserved sequence elements (CSEs) to both the left and the right of the cleaved bonds, 16 bp altogether with some variability tolerated. In vitro, however, single-strand nicks in the lower strand predominate at sites containing only the left-side CSE that determines the precise position of lower strand nicks. Upper strand nick positions vary both in vivo and in vitro. A 24 bp substrate was nicked with the same precision as in longer substrates, showing that the conserved sequence suffices for precise nicking by EndoII. Using DNA ligase in vitro, we found that EndoII nicked both strands simultaneously at an in vivo-favoured site but not at an in vitro-favoured site. This indicates that the right-side CSE at in vivo-favoured sites is important for simultaneous nicking of both strands, generating double-strand cleavage. Separate analysis of the two strands following in vitro digestion at two in vitro-favoured sites showed that EndoII nicked the lower strand about 1.5-fold faster than the upper strand. In addition, the upper and lower strands were nicked independently of each other, seldom resulting in double-strand cleavage. Thus, cleavage by EndoII is the fortuitous outcome of two separate nicking events.

Bacteriophage T4 genome

Phage T4 has provided countless contributions to the paradigms of genetics and biochemistry. Its complete genome sequence of 168,903 bp encodes about 300 gene products. T4 biology and its genomic sequence provide the best-understood model for modern functional genomics and proteomics. Variations on gene expression, including overlapping genes, internal translation initiation, spliced genes, translational bypassing, and RNA processing, alert us to the caveats of purely computational methods. The T4 transcriptional pattern reflects its dependence on the host RNA polymerase and the use of phage-encoded proteins that sequentially modify RNA polymerase; transcriptional activator proteins, a phage sigma factor, anti-sigma, and sigma decoy proteins also act to specify early, middle, and late promoter recognition. Posttranscriptional controls by T4 provide excellent systems for the study of RNA-dependent processes, particularly at the structural level. The redundancy of DNA replication and recombination systems of T4 reveals how phage and other genomes are stably replicated and repaired in different environments, providing insight into genome evolution and adaptations to new hosts and growth environments. Moreover, genomic sequence analysis has provided new insights into tail fiber variation, lysis, gene duplications, and membrane localization of proteins, while high-resolution structural determination of the "cell-puncturing device," combined with the three-dimensional image reconstruction of the baseplate, has revealed the mechanism of penetration during infection. Despite these advances, nearly 130 potential T4 genes remain uncharacterized. Current phage-sequencing initiatives are now revealing the similarities and differences among members of the T4 family, including those that infect bacteria other than Escherichia coli. T4 functional genomics will aid in the interpretation of these newly sequenced T4-related genomes and in broadening our understanding of the complex evolution and ecology of phages-the most abundant and among the most ancient biological entities on Earth.

Sequence-specific cleavage by bacteriophage T4 endonuclease II in vitro

The 136 codon (408 bp) denA gene encoding endonuclease II (Endoll) of bacteriophage T4 was unambiguously identified through sequencing and subsequent cloning. Endoll prepared from cloned DNA through coupled in vitro transcription-translation nicked and cut DNA in vitro in a sequence-specific manner. In vitro (and in vivo), the bottom strand was nicked between the first and second base pair to the right of a top-strand CCGC motif shared by favoured in vitro and in vivo cleavage sites; top-strand cleavage positions varied. To the right of the cleavage position, favoured in vitro sites lacked a sequence element conserved at favoured in vivo sites. In pBR322 DNA, the sites cleaved in vivo as previously described were also cleaved in vitro, but in vitro additional sites were nicked or cleaved and the preference for individual sites was different. Also, different from the in vivo reaction, nicking was more frequent than ds cutting; in many copies of a ds cleavage site, only the bottom strand was nicked in vitro. A model is discussed in which sequential nicking of the two strands, and different factors influencing bottom-strand nicking and top-strand nicking, can explain the differences between the in vitro and the in vivo reaction.

Endonuclease II of coliphage T4: a recombinase disguised as a restriction endonuclease?

EndoII shares with restriction endonucleases the property of cleaving foreign DNA while leaving the endonuclease-encoding genome intact, ensuring the survival of one DNA species in the cell. In addition, in vivo EndoII cleaves a specific DNA sequence and cleavage is context dependent. These context effects extend over at least 1000 bp, largely limiting cleavage to once within this distance. Like homing endonucleases, in vivo EndoII recognizes a long, asymmetric and degenerate consensus sequence which has two distinct parts. Recognition of one part of the consensus sequence involves base-specific bonds, and recognition of the other involves sequence-dependent helical structure. EndoII fulfills an obvious short-term survival role in ensuring the dominance of phage DNA in an infected cell, but may also have a long-term evolutionary role, producing gene-size fragments of foreign DNA to be enrolled in the phage genetic repertoire.