Production and expression of dTMP-enriched DNA of bacteriophage SP15

The Sequence of Two Bacteriophages with Hypermodified Bases Reveals Novel Phage-Host Interactions

Bacteriophages SP-15 and ΦW-14 are members of the Myoviridae infecting Bacillus subtilis and Delftia (formerly Pseudomonas) acidovorans, respectively. What links them is that in both cases, approximately 50% of the thymine residues are replaced by hypermodified bases. The consequence of this is that the physico-chemical properties of the DNA are radically altered (melting temperature (Tm), buoyant density and susceptibility to restriction endonucleases). Using 454 pyrosequencing technology, we sequenced the genomes of both viruses. Phage ΦW-14 possesses a 157-kb genome (56.3% GC) specifying 236 proteins, while SP-15 is larger at 222 kb (38.6 mol % G + C) and encodes 318 proteins. In both cases, the phages can be considered genomic singletons since they do not possess BLASTn homologs. While no obvious genes were identified as being responsible for the modified base in ΦW-14, SP-15 contains a cluster of genes obviously involved in carbohydrate metabolism.

Biosynthesis and Function of Modified Bases in Bacteria and Their Viruses

Naturally occurring modification of the canonical A, G, C, and T bases can be found in the DNA of cellular organisms and viruses from all domains of life. Bacterial viruses (bacteriophages) are a particularly rich but still underexploited source of such modified variant nucleotides. The modifications conserve the coding and base-pairing functions of DNA, but add regulatory and protective functions. In prokaryotes, modified bases appear primarily to be part of an arms race between bacteriophages (and other genomic parasites) and their hosts, although, as in eukaryotes, some modifications have been adapted to convey epigenetic information. The first half of this review catalogs the identification and diversity of DNA modifications found in bacteria and bacteriophages. What is known about the biogenesis, context, and function of these modifications are also described. The second part of the review places these DNA modifications in the context of the arms race between bacteria and bacteriophages. It focuses particularly on the defense and counter-defense strategies that turn on direct recognition of the presence of a modified base. Where modification has been shown to affect other DNA transactions, such as expression and chromosome segregation, that is summarized, with reference to recent reviews.

DNA Synthesis and Gene Expression in Bacillus subtilis Infected with Wild-Type and Hypermodification-Defective Bacteriophage SP10

A hypermodified base (Y-Thy) replaces 20% of the thymine (Thy) in mature DNA of Bacillus subtilis phage SP10. Two noncomplementing hypermodification-defective (hmd) mutants are described. At 30 degrees C, hmd phage carried out a normal program, but at temperatures of >/=37 degrees C, the infection process was nonproductive. When cells were infected at 37 degrees C with hmd phage, DNA synthesis started at its usual time (12 min), proceeded at about half the normal rate for 6 to 8 min, and then stopped or declined manyfold. All, or nearly all, of the DNA made under hmd conditions consisted of fully hypermodified parental DNA strands H-bonded to unhypermodified nascent strands. The reduced levels of DNA synthesis observed under hmd conditions were accompanied by weak expression of late genes. A sucrose gradient analysis of SP10 hmd(+) replicating DNA intermediates was made. Two intermediates, called VG and F, were identified. VF consisted of condensed DNA complexed to protein; VF also contained negatively supercoiled domains covalently joined to relaxed regions. F was composed of linear concatenates from which mature DNA was cleaved. None of those intermediates was evident in cells infected at 37 degrees C with hmd phage. Shiftup experiments were performed wherein cells infected with hmd phage at 30 degrees C were shifted to 37 degrees C at a time when replication was well under way. DNA synthesis stopped or declined manyfold 10 min after shiftup. The hmd DNA made after shiftup was conserved as a form sedimentationally equivalent to the F intermediate, but little mature DNA was evident. It is proposed that Y-Thy is required for replication and DNA maturation because certain key proteins involved with these processes interact preferentially with hypermodified DNA.

Polymer-level synthesis of oxopyrimidine deoxynucleotides by Bacillus subtilis phage SP10: characterization of modification-defective mutants

Bacillus subtilis phage SP10 DNA has two oxopyrimidines, thymidine 5'-monophosphate (dTMP) and its hypermodified analog (YdTMP). Published data suggest that both are synthesized by postreplicational modification of 5-hydroxymethyldeoxyuridylate (HOMedUMP) in nascent DNA by the following pathway: HOMedUMP----PPOMedUMP----dTMP (85%) or YdTMP (15%); PPOMedUMP is 5-(hydroxymethyl-O-pyrophosphoryl)deoxyuridylate, the pyrophosphoric acid ester of the C5CH2OH function of HOMedUMP. This paper describes aberrant DNAs synthesized at nonpermissive temperatures by a complementary series of heat-sensitive, modification-defective (mod) mutants. Collectively, these mutants encompass the major steps in the complete modification of nascent SP10 DNA. DNA produced by modA phage retains HOMedUMP as its sole oxopyrimidine, implying that (i) this mutant is defective in the pyrophosphorylation step and (ii) formation of PPOMedUMP is required for any further modification. Furthermore, studies with double mutants indicated that modA is epistatic for all other mod mutants, which supports the hypothesis that modA controls the earliest step in the modification pathway. Since their DNAs contain no YdTMP, modC and modD are defective in hypermodification (i.e., PPOMedUMP----YdTMP). However, dTMP occupies the entire oxopyrimidine fraction of modC DNA, whereas modD DNA has a normal dTMP content, but the now-missing YdTMP is replaced by either PPOMedUMP or a byproduct of abortive hypermodification. It is proposed that the modD mutants are defective in the catalytic aspects of hypermodification and that modC are defective in some regulatory function that promotes hypermodification at the expense of reductive modification (i.e., PPOMedUMP----dTMP). Reductive modification is defective in modB phage, as evidenced by the absence of dTMP. In contrast to the others, modB DNA has a complex oxopyrimidine content: HOMedUMP, ca. 30%; PPOMedUMP, ca. 40%; and YdTMP, ca. 30%. The expanded level of YdTMP suggests that at certain sites, reductive modification and hypermodification are competing reactions. Interestingly, the PPOMedUMP content of modB DNA seemingly reflects the maximum degree to which phage DNA can be pyrophosphorylated, since the loss of YdTMP from modBmodC and modBmodD DNAs results in a unilateral increase in HOMedUMP content.

DNA synthesis in Pseudomonas acidovorans infected with mutants of bacteriophage phi W-14 defective in the synthesis of alpha-putrescinylthymine

Normal levels of the hypermodified pyrimidine, alpha-putrescinylthymine, which is formed from hydhydroxymethyluracil at the polynucleotide level (Maltman et al., J. Virol. 34:354-359, 1984), are not required in bacteriophage luminal diameterW-14 DNA for the DNA to serve as a replicative template in luminal diameterW-14-infected cells.

Selective protection of 5' ... GGCC ... 3' and 5' ... GCNGC ... 3' sequences by the hypermodified oxopyrimidine in Bacillus subtilis bacteriophage SP10 DNA

The DNA of Bacillus subtilis bacteriophage SP10 is partially resistant to cleavage and methylation in vitro by restriction enzyme R . BsuRI and its cognate methylase even though greater than 20 copies of the target sequence, 5' ... GGCC ... 3', are present on the phage genome. YThy, a hypermodified oxopyrimidine that replaces a fraction of the thymine residues in SP10 DNA, was responsible for this protection, since YThy-free DNA was no longer resistant. Sites that were normally resistant could nevertheless be cleaved or methylated in vitro if the salt concentration was reduced or dimethyl sulfoxide was added to the reaction buffer. Analysis of the termini produced by cleavage suggested that resistant sites occurred in the sequence 5' ... GGCC-YThy ... 3', whereas sensitive sites, of which there were only two per genome, occurred in the sequence 5' ... GGCCG ... 3'. These in vitro results provide an explanation for the in vivo resistance of SP10 to restriction-modification by B. subtilis R. They also suggest ways in which the presence of the atypical base YThy in regions that flank the target might upset critical DNA-enzyme interactions necessary to locate and recognize the specific site of cleavage or methylation. YThy also strongly protected 5' ... GCNGC ... 3' (R . Fnu4HI) sequences on SP10 DNA, but the biological relevance of this protection is unclear.

Synthesis of deoxythymidylate and the unusual deoxynucleotide in mature DNA of Bacillus subtilis bacteriophage SP10 occurs by postreplicational modification of 5-hydroxymethyldeoxyuridylate

Mature DNA of Bacillus subtilis W23 phage SP10 contains a hypermodified nucleotide (YdTMP) that replaces ca. 20% of the DTMP. SP10 DNA was pulse-labeled for 1 min at 20 degrees C with 32Pi. Among the oxopyrimidine nucleotides, virtually all of the radioactivity was recovered as 5-hydroxymethyldeoxyuridylate (HMdUMP). During the subsequent chase, radioactivity was lost from HMdUMP and recovered as YdTMP. At 37 degrees C, exogenous [6-3H]5-hydroxymethyldeoxyuridine (HMdUrd) was incorporated into SP10 DNA. Label administered as HMdUrd was phosphorylated to HMdUTP in the infected cells, but all radioactivity was recovered from SP10 DNA as YdTMP and dTMP. Two heat-sensitive mutants defective in hypermodification of SP10 DNA are described. In one mutant, HMdUMP replaces YdTMP in DNA. The other mutant generates a DNA containing a novel deoxynucleotide in place of YdTMP. The novel deoxynucleotide seems to consist of PPi esterified to the 5-hydroxymethyl function of HMdUMP (PP-HMdUMP). Both mutants make normal amounts of dTMP. The data are discussed in terms of the following conclusions. (i) Both oxopyrimidine nucleotides in mature SP10 DNA are derived by postreplicative modification of HMdUMP in nascent DNA. (ii) PP-HMdUMP is an intermediate that facilitate formation of a putative exocyclic methylene intermediate which receives the hypermodification. It is also argued that PP-HMdUMP and the same exocyclic methylene intermediate could serve as intermediates in reductive modification to dTMP. (iii) YdTMP is not an intermediate in the formation of dTMP, and reductive modification proceeds independently of hypermodification.