Acridine-resistance in phage T4D

Genomic characterization of key bacteriophages to formulate the potential biocontrol agent to combat enteric pathogenic bacteria

Combating bacterial pathogens has become a global concern especially when the antibiotics and chemical agents are failing to control the spread due to its resistance. Bacteriophages act as a safe biocontrol agent by selectively lysing the bacterial pathogens without affecting the natural beneficial microflora. The present study describes the screening of prominent enteric pathogens NDK1, NDK2, NDK3, and NDK4 (Escherichia, Klebsiella, Enterobacter, and Serratia) mostly observed in domestic wastewater; against which KNP1, KNP2, KNP3, and KNP4 phages were isolated. To analyze their potential role in eradicating enteric pathogens and toxicity issue, these bacteriophages were sequenced using next-generation sequencing and characterized based on its genomic content. The isolated bacteriophages were homologous to Escherichia phage (KNP1), Klebsiella phage (KNP2), Enterobacter phage (KNP3), Serratia phage (KNP4), and belonged to Myoviridae family of Caudovirales except for the unclassified KNP4 phage. Draft genome analysis revealed the presence of lytic enzymes such as holing and lysozyme in KNP1 phage, endolysin in KNP2 phage, and endopeptidase with holin in KNP3 phage. The absence of any lysogenic and virulent genes makes this bacteriophage suitable candidate for preparation of phage cocktail to combat the pathogens present in wastewater. However, KNP4 contained a virulent gene rendering it unsuitable to be used as a biocontrol agent. These findings make the phages (KNP1-KNP3) as a promising alternative for the biocontrol of pathogens in wastewater which is the main culprit to spread these dominated pathogens in different natural water bodies. This study also necessitates for genomic screening of bacteriophages for lysogenic and virulence genes prior to its use as a biocontrol agent.

What Can We Learn from a Metagenomic Analysis of a Georgian Bacteriophage Cocktail?

Phage therapy, a practice widespread in Eastern Europe, has untapped potential in the combat against antibiotic-resistant bacterial infections. However, technology transfer to Western medicine is proving challenging. Bioinformatics analysis could help to facilitate this endeavor. In the present study, the Intesti phage cocktail, a key commercial product of the Eliava Institute, Georgia, has been tested on a selection of bacterial strains, sequenced as a metagenomic sample, de novo assembled and analyzed by bioinformatics methods. Furthermore, eight bacterial host strains were infected with the cocktail and the resulting lysates sequenced and compared to the unamplified cocktail. The analysis identified 23 major phage clusters in different abundances in the cocktail, among those clusters related to the ICTV genera T4likevirus, T5likevirus, T7likevirus, Chilikevirus and Twortlikevirus, as well as a cluster that was quite distant to the database sequences and a novel Proteus phage cluster. Examination of the depth of coverage showed the clusters to have different abundances within the cocktail. The cocktail was found to be composed primarily of Myoviridae (35%) and Siphoviridae (32%), with Podoviridae being a minority (15%). No undesirable genes were found.

Compression of the DNA substrate by a viral packaging motor is supported by removal of intercalating dye during translocation

Viral genome packaging into capsids is powered by high-force-generating motor proteins. In the presence of all packaging components, ATP-powered translocation in vitro expels all detectable tightly bound YOYO-1 dye from packaged short dsDNA substrates and removes all aminoacridine dye from packaged genomic DNA in vivo. In contrast, in the absence of packaging, the purified T4 packaging ATPase alone can only remove up to ∼1/3 of DNA-bound intercalating YOYO-1 dye molecules in the presence of ATP or ATP-γ-S. In sufficient concentration, intercalating dyes arrest packaging, but rare terminase mutations confer resistance. These distant mutations are highly interdependent in acquiring function and resistance and likely mark motor contact points with the translocating DNA. In stalled Y-DNAs, FRET has shown a decrease in distance from the phage T4 terminase C terminus to portal consistent with a linear motor, and in the Y-stem DNA compression between closely positioned dye pairs. Taken together with prior FRET studies of conformational changes in stalled Y-DNAs, removal of intercalating compounds by the packaging motor demonstrates conformational change in DNA during normal translocation at low packaging resistance and supports a proposed linear "DNA crunching" or torsional compression motor mechanism involving a transient grip-and-release structural change in B form DNA.

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.

A species barrier between bacteriophages T2 and T4: exclusion, join-copy and join-cut-copy recombination and mutagenesis in the dCTPase genes

Bacteriophage T2 alleles are excluded in crosses between T2 and T4 because of genetic isolation between these two virus species. The severity of exclusion varies in different genes, with gene 56, encoding an essential dCT(D)Pase/dUT(D)Pase of these phages, being most strongly affected. To investigate reasons for such strong exclusion, we have (1) sequenced the T2 gene 56 and an adjacent region, (2) compared the sequence with the corresponding T4 DNA, (3) constructed chimeric phages in which T2 and T4 sequences of this region are recombined, and (4) tested complementation, recombination, and exclusion with gene 56 cloned in a plasmid and in the chimeric phages in Escherichia coli CR63, in which growth of wild-type T2 is not restricted by T4. Our results argue against a role of the dCTPase protein in this exclusion and implicate instead DNA sequence differences as major contributors to the apparent species barrier. This sequence divergence exhibits a remarkable pattern: a major heterologous sequence counter-clockwise from gene 56 (and downstream of the gene 56 transcripts) replaces in T2 DNA the T4 gene 69. Gene 56 base sequences bordering this substituted region are significantly different, whereas sequences of the dam genes, adjacent in the clockwise direction, are similar in T2 and in T4. The gene 56 sequence differences can best be explained by multiple compensating frameshifts and base substitutions, which result in T2 and T4 dCTPases whose amino acid sequences and functions remain similar. Based on these findings we propose a model for the evolution of multiple sequence differences concomitant with the substitution of an adjacent gene by foreign DNA: invasion by the single-stranded segments of foreign DNA, nucleated from a short DNA sequence that was complementary by chance, has triggered recombination-dependent replication by "join-copy" and "join-cut-copy" pathways that are known to operate in the T-even phages and are implicated in other organisms as well. This invasion, accompanied by heteroduplex formation between partially similar sequences, and perhaps subsequent partial heteroduplex repair, simultaneously substituted T4 gene 69 for foreign sequences and scrambled the sequence of the dCTPase gene 56. We suggest that similar mechanisms can mobilize DNA segments for horizontal transfer without necessarily requiring transposase or site-specific recombination functions.

The spectrum of acridine resistant mutants of bacteriophage T4 reveals cryptic effects of the tsL141 DNA polymerase allele on spontaneous mutagenesis

Mutations in the ac gene of bacteriophage T4 confer resistance to acridine-inhibition of phage development. Previous studies had localized the ac gene region; we show that inactivation of T4 Open Reading Frame 52.2 confers the Acr phenotype. Thus, 52.2 is ac. The resistance mechanism is unknown. The ac gene provides a convenient forward mutagenesis assay. Its compact size (156 bp) simplifies mutant sequencing and diverse mutant types are found: base substitutions leading to missense or nonsense codons, in-frame deletions or duplications within the coding sequence, deletion or duplication frameshifts, insertions, complex mutations, and large deletions extending into neighboring sequences. Comparisons of spontaneous mutagenesis between phages bearing the wild-type or tsL141 alleles of DNA polymerase demonstrate that the impact of the mutant polymerase is cryptic when total spontaneous mutant frequencies are compared, but the DNA sequences of the ac mutants reveal a substantial alteration of fidelity by the mutant polymerase. The patterns of base substitution mutagenesis suggest that some site-specific mutation rate effects may reflect hotspots for mutagenesis arising by different mechanisms. A new class of spontaneous duplication mutations, having sequences inconsistent with misaligned pairing models, but consistent with nick-processing errors, has been identified at a hotspot in ac.

Mechanisms of spontaneous and induced frameshift mutation in bacteriophage T4

Frequencies of spontaneous and proflavine-induced frameshift mutations increased dramatically as a function of the number of reiterated base pairs at each of two sites in the lysozyme gene of bacteriophage T4. At each site, proflavine induces addition mutations more frequently than deletion mutations. We confirm that the steroidal diamine, irehdiamine A, induces frameshift addition mutations. At sites of reiterated bases, we propose that base pairing is misaligned adjacent to a gap. The misaligned configuration is stabilized by the stacking of mutagen molecules around the extrahelical base, forming a sandwich. Proflavine induces addition mutations efficiently at a site without any reiterated bases. Mutagenesis at such sites may be due to mutagen-induced stuttering of the replication complex.

Model for DNA packaging into bacteriophage T4 heads

The mechanism of DNA packaging into bacteriophage T4 heads in vivo was investigated by glucosylation of hydroxymethylcytosine residues in a conditionally glucose-deficient host. Cytoplasmic DNA associated with partially packaged ts49 heads can be fully glucosylated, whereas DNA already packaged into these heads is shown to be resistant to glucosylation. After temperature shift and completion of arrested packaging into the reversible temperature-sensitive ts49 head, the structure of the DNA in the mature ts49 phage was investigated by restriction enzyme digestion, autoradiography, and other techniques. Such mature DNA appears to be fully glucosylated along part of its length and nonglucosylated on the remainder. Its structure suggests that the DNA is run into the head linearly and unidirectionally from one mature end and that there is little sequence specificity in that portion of the T4 DNA which first enters the capsid. This technique should be useful in investigation of the three-dimensional structure of first- and last-packaged DNA within the head; preliminary studies including autoradiography of osmotically shocked phage suggest that the DNA which first enters the head is deposited toward the center of the capsid and that the end of the DNA which first enters the head exits first upon injection. In conjunction with studies of the structure of condensed DNA, the positions and functions of T4 capsid proteins in DNA packaging, and the order of T4 packaging functions [Earnshaw and Harrison, Nature (London) 268:598-602, 1977; Hsiao and Black, Proc. Natl. Acad. Sci. U.S.A. 74:3652-3656, 1977; Müller-Salamin et al., J. Virol. 24:121-134, 1977; Richards et al., J. Mol. Biol. 78:255-259, 1973], the features described above suggest the following model: the first DNA end is fixed to the proximal apex of the head at p20 and the DNA is then pumped into the head enzymatically by proteins (p20 + p17) which induce torsion in the DNA molecule.

The inhibition of bacteriophage DNA injection by dyes bound to the DNA

A delay ( approximately 10 min) in the appearance of intracellular phage is caused by preincubating the infecting phage T4o(1) in proflavine, acridine orange, or ethidium, but not polyamines. No significant delay in attachment is observed. Apparently the presence of the dye is required inside the permeability barrier of the phage at the time of infection. The effect of proflavine is reduced in the presence of polyamines, suggesting that the active site is on DNA. The phage-host complex is sensitive to shear if the infecting phage have been incubated in proflavine or ethidium, indicating that the completion of DNA injection is delayed. Finally no partially injected complexes could be detected after shearing, which suggests that most of the delay occurs near the beginning of the injection process.