The bacteriophage T4 gene mrh whose product inhibits late T4 gene expression in an Escherichia coli rpoH (sigma 32) mutant

Systemic Expression, Purification, and Initial Structural Characterization of Bacteriophage T4 Proteins Without Known Structure Homologs

Bacteriophage T4 of Escherichia coli is one of the most studied phages. Research into it has led to numerous contributions to phage biology and biochemistry. Coding about 300 gene products, this double-stranded DNA virus is the best-understood model in phage study and modern genomics and proteomics. Ranging from viral RNA polymerase, commonly found in phages, to thymidylate synthase, whose mRNA requires eukaryotic-like self-splicing, its gene products provide a pool of fine examples for phage research. However, there are still up to 130 gene products that remain poorly characterized despite being one of the most-studied model phages. With the recent advancement of cryo-electron microscopy, we have a glimpse of the virion and the structural proteins that present in the final assembly. Unfortunately, proteins participating in other stages of phage development are absent. Here, we report our systemic analysis on 22 of these structurally uncharacterized proteins, of which none has a known homologous structure due to the low sequence homology to published structures and does not belong to the category of viral structural protein. Using NMR spectroscopy and cryo-EM, we provided a set of preliminary structural information for some of these proteins including NMR backbone assignment for Cef. Our findings pave the way for structural determination for the phage proteins, whose sequences are mainly conserved among phages. While this work provides the foundation for structural determinations of proteins like Gp57B, Cef, Y04L, and Mrh, other in vitro studies would also benefit from the high yield expression of these proteins.

The Bacteriophage T4 MotB Protein, a DNA-Binding Protein, Improves Phage Fitness

The lytic bacteriophage T4 employs multiple phage-encoded early proteins to takeover the Escherichia coli host. However, the functions of many of these proteins are not known. In this study, we have characterized the T4 early gene motB, located in a dispensable region of the T4 genome. We show that heterologous production of MotB is highly toxic to E. coli, resulting in cell death or growth arrest depending on the strain and that the presence of motB increases T4 burst size 2-fold. Previous work suggested that motB affects middle gene expression, but our transcriptome analyses of T4 motB^am vs. T4 wt infections reveal that only a few late genes are mildly impaired at 5 min post-infection, and expression of early and middle genes is unaffected. We find that MotB is a DNA-binding protein that binds both unmodified host and T4 modified [(glucosylated, hydroxymethylated-5 cytosine, (GHme-C)] DNA with no detectable sequence specificity. Interestingly, MotB copurifies with the host histone-like proteins, H-NS and StpA, either directly or through cobinding to DNA. We show that H-NS also binds modified T4 DNA and speculate that MotB may alter how H-NS interacts with T4 DNA, host DNA, or both, thereby improving the growth of the phage.

The role of interactions between phage and bacterial proteins within the infected cell: a diverse and puzzling interactome

Interactions between bacteriophage proteins and bacterial proteins are important for efficient infection of the host cell. The phage proteins involved in these bacteriophage-host interactions are often produced immediately after infection. A survey of the available set of published bacteriophage-host interactions reveals the targeted host proteins are inhibited, activated or functionally redirected by the phage protein. These interactions protect the bacteriophage from bacterial defence mechanisms or adapt the host-cell metabolism to establish an efficient infection cycle. Regrettably, a large majority of bacteriophage early proteins lack any identified function. Recent research into the antibacterial potential of bacteriophage-host interactions indicates that phage early proteins seem to target a wide variety of processes in the host cell - many of them non-essential. Since a clear understanding of such interactions may become important for regulations involving phage therapy and in biotechnological applications, increased scientific emphasis on the biological elucidation of such proteins is warranted.

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.

Two new early bacteriophage T4 genes, repEA and repEB, that are important for DNA replication initiated from origin E

Two new, small, early bacteriophage T4 genes, repEA and repEB, located within the origin E (oriE) region of T4 DNA replication, affect functioning of this origin. An important and unusual property of the oriE region is that it is transcribed at early and late periods after infection, but in opposite directions (from complementary DNA strands). The early transcripts are mRNAs for RepEA and RepEB proteins, and they can serve as primers for leading-strand DNA synthesis. The late transcripts, which are genuine antisense RNAs for the early transcripts, direct synthesis of virion components. Because the T4 genome contains several origins, and because recombination can bypass a primase requirement for retrograde synthesis, neither defects in a single origin nor primase deficiencies are lethal in T4 (Mosig et al., FEMS Microbiol. Rev. 17:83-98, 1995). Therefore, repEA and repEB were expected and found to be important for T4 DNA replication only when activities of other origins were reduced. To investigate the in vivo roles of the two repE genes, we constructed nonsense mutations in each of them and combined them with the motA mutation sip1 that greatly reduces initiation from other origins. As expected, T4 DNA synthesis and progeny production were severely reduced in the double mutants as compared with the single motA mutant, but early transcription of oriE was reduced neither in the motA nor in the repE mutants. Moreover, residual DNA replication and growth of the double mutants were different at different temperatures, suggesting different functions for repEA and repEB. We surmise that the different structures and protein requirements for functioning of the different origins enhance the flexibility of T4 to adapt to varied growth conditions, and we expect that different origins in other organisms with multiorigin chromosomes might differ in structure and function for similar reasons.

Sigma competition: the contest between bacteriophage T4 middle and late transcription

In bacterial transcription, diverse sigma-family promoter recognition proteins compete for a common RNA polymerase core. Bacteriophage T4 infection ultimately reduces this competition to a duel between activated viral middle and enhanced late transcription, involving two sigma proteins, two phage-encoded activator proteins and two phage-specific co-activators. This competition has been analyzed in vitro, and the relative abundances in T4-infected Escherichia coli of the participating proteins have been measured. Activated late transcription holds an advantage over activated middle transcription, especially at higher ionic strength. This advantage is further compounded by ADP-ribosylation of the RNA polymerase alpha subunits, and by the phage-specific, RNA polymerase core-bound RpbA subunit. The largest contribution to the middle-late competition is made by gp55, the late sigma factor, but not enough of gp55 is produced during T4 infection to shut off middle transcription by direct competition with sigma(70). AsiA, the originally identified anti-sigma protein is not a major determinant of middle-late competition.

Several new bacteriophage T4 genes, mapped by sequencing deletion endpoints between genes 56 (dCTPase) and dda (a DNA-dependent ATPase-helicase) modulate transcription

We have analyzed DNA of wild-type T4 and of 13 independent large viable deletions isolated by Homyk and Weil (Virology 61 (1974) 505-523) and by Little (Virology 53 (1973) 47-59), by sequencing, cloning, and expression studies. The deletions can be explained by illegitimate recombination between short (4- to 15-bp) ectopic repeats. In four deletions, adjacent regions are partially homologous, and in at least one of them, the base adjacent to the overlap was deleted during recombination. The sequence 5'-GGGC, which has not been associated with T4 deletions in other map regions, occurs within three repeats, and near the repeats in four more of the 13 deletions. Five previously named genes, 69, soc, mrh, modA, and dda were mapped relative to the deletion endpoints. Nine additional ORFs were found interspersed between them. One of these shares some similarities with mrh (modulates rpoH; Frazier and Mosig, Gene 88 (1990) 7-14), and another one resembles modA (coding for an ADP-ribosyl-transferase that modifies RNA polymerase alpha subunits, Skórko et al., Eur. J. Biochem. 79 (1977) 55-66) respectively. We found that the host's heat shock sigma factor, sigma32, is phosphorylated, and that Mrh protein modulates this phosphorylation. The ORF dda.9 downstream of mrh has a patchy similarity with conserved C-terminal segments (motifs) of sigma32; therefore, we call it srh. Another ORF, dda.2 located between modA and dda, shares sequence similarity with sigma70, and we call it srd. We consider the possibility that Srh and Srd act as decoys for sigma32, or sigma70, respectively. Expression of several of the ORFs from cloned DNA appears to be toxic to the host bacteria. Mutant clones only could be constructed from gene 69 and from modA. Moreover, dda.2 (srd)-containing bacteria grow extremely slowly, and they form filaments in liquid cultures. Clones carrying mrh and srh show less severe filamentation. Our results highlight the importance of 'non-essential' genes for phage development and evolution.

Two bacteriophage T4 base plate genes (25 and 26) and the DNA repair gene uvsY belong to spatially and temporally overlapping transcription units

The bacteriophage T4 DNA recombination-repair gene uvsY located at or near an origin of DNA replication and adjacent to the late base plate genes 25 and 26. Our present results reveal a complex transcription pattern in the region encompassing these genes. Most significantly, uvsY and two ORFs, downstream of it, all of which are transcribed from a middle promoter before the onset of DNA replication, are also part of a larger late transcription unit which includes the base plate genes 25 and 26. The late genes 25 and 26 are transcribed not only late, but also early from one or several early promoters further upstream. Translation, however, is inhibited by secondary structures which sequester the ribosome binding site in the early transcript. We discuss possible advantages of these transcriptional patterns for T4 DNA recombination, replication, and repair. The predicted and in vivo-expressed 23.9-kDa product of gene 26 is smaller than the reported size of gene 26 protein isolated from base plates, suggesting that nascent gp26 might be processed to a larger protein during assembly.