IS801, an insertion sequence element isolated from Pseudomonas syringae pathovar phaseolicola

A transposable element, designated IS801, was isolated from strain LR781 of Pseudomonas syringae pathovar phaseolicola in two independent events using the entrapment plasmid, pUCD800. IS801 is 1517 base pairs in length and contains open reading frames that potentially encode proteins of 311 and 172 amino acids, as well as smaller proteins. Unlike most other prokaryotic transposable elements, IS801 lacks terminal repeats. Sequence analysis revealed two target pentamers for IS801 insertion that differ by one base pair. One copy of IS801 generated a perfect duplication of its target, TGAAC. The second copy of IS801 was flanked by the target, TGGAC, at one end, and TGAAC at the other end. A third copy of IS801 was cloned from pMMC7105, an indigenous plasmid of strain LR781, and it was flanked by copies of the pentamer TGAAC.

Purified phi 6 nucleocapsids are capable of productive infection of host cells with partially disrupted outer membranes

Purified nucleocapsids of bacteriophage phi 6, lacking the phage lipid envelope, are unable to infect intact Pseudomonas syringae host cells. A method for studying the process by which a naked virus particle, the phi 6 nucleocapsid, penetrates the host cytoplasmic membrane was developed. Host cells were rendered competent for nucleocapsid infection by treatment with repeated washings with salt and sucrose and the subsequent addition of lysozyme. This treatment disrupts the outer membrane, permitting the nucleocapsid to reach the cytoplasmic membrane and to infect the cell. The nucleocapsid infection is blocked by monoclonal antibodies raised against the nucleocapsid shell protein P8.

The nucleocapsid of bacteriophage phi 6 penetrates the host cytoplasmic membrane

Bacteriophage phi 6 infects its host, the Gram-negative bacterium Pseudomonas syringae, by a protein-targeted fusion of the virus envelope with the host outer membrane. In this investigation we present results suggesting that the phage nucleocapsid penetrates the host cytoplasmic membrane via a membrane invagination and an intracellular vesicle. This indicates that the prokaryotic plasma membrane might be more dynamic and have more common features with eukaryotic membrane systems than previously expected. Most of the nucleocapsid surface lattice protein is degraded in the cell, and the nucleocapsid core particle containing the viral dsRNA segments and the proteins necessary for the viral RNA polymerase activity can be isolated from the infected cells. The penetration is dependent on the energized state of the host cytoplasmic membrane. About 25% of the entering core particles are re-used in the progeny viruses.

Nucleotide sequence of the large double-stranded RNA segment of bacteriophage phi 6: genes specifying the viral replicase and transcriptase

The genome of the lipid-containing bacteriophage phi 6 contains three segments of double-stranded RNA. We determined the nucleotide sequence of cDNA derived from the largest RNA segment (L). This segment specifies the procapsid proteins necessary for transcription and replication of the phi 6 genome. The coding sequences of the four proteins on this segment were identified on the basis of size and the correlation of predicted N-terminal amino acid sequences with those found through analysis of isolated proteins. This report completes the sequence analysis of phi 6. This constitutes the first complete sequence of a double-stranded RNA genome virus.

Nucleotide sequence of the middle dsRNA segment of bacteriophage phi 6: placement of the genes of membrane-associated proteins

The genome of the lipid-containing bacteriophage phi 6 contains three segments of double-stranded RNA. We have determined the nucleotide sequence of cDNA derived from the middle-size RNA segment. The coding sequences of three proteins on this segment were identified on the basis of size and the correlation of predicted N-terminal amino acid sequences with those found through the analysis of isolated proteins. In contrast to our results with the small phi 6 dsRNA segment, the open reading frames are not tightly clustered. The homologous terminal noncoding regions between the middle and small dsRNA segments are found to be more extensive than RNA sequencing had previously indicated.

Membrane fusion in prokaryotes: bacteriophage phi 6 membrane fuses with the Pseudomonas syringae outer membrane

Protein-triggered membrane fusion in the prokaryotic system is described using the lipid-containing enveloped bacterial virus phi 6 and its host, the Gram-negative bacterium Pseudomonas syringae. Bacteriophage particles can be fused to form multiple particles where two or more nucleocapsids are surrounded by a single membrane vesicle with a volume proportional to the number of fused particles. For fusion to occur, a fusogenic protein is required in the membrane of the participating phage particles. Upon infection of the host cell, fusion of the viral membrane with the bacterial membrane takes place without leakage of the periplasmic enzyme alkaline phosphatase to the extracellular supernatant. There is a time-dependent mixing of fluorescent phage phospholipids with the bacterial membrane lipids between 5 and 20 min post-infection. The phage membrane proteins and phospholipids co-purify with the bacterial outer membrane of infected cells. The fusion is independent of divalent cations and pH, resembling Sendai virus fusion with the plasma membrane. This is the first targeted, protein-dependent fusion event described in prokaryotes.

The causal agent of halo blight in bean, Pseudomonas syringae pv. phaseolicola, attaches to stomata via its pili

The phytopathogenic pseudomonad Pseudomonas syringae pv. phaseolicola causes halo blight of bean (Phaseolus vulgaris L.). Initiation of infection depends on the ability of the cells to adhere to the target cell surface. P. syringae pv. phaseolicola expresses pili, which are the receptors of the lipid-containing dsRNA bacteriophage phi 6. phi 6-resistant bacterial strains can be divided into different piliation types. It was possible to show that the adhesion of the bacteria onto plant cell surface was dependent on the pili. Non-piliated bacterial stains showed a much lower adherence to the leaf surface than strains expressing phi 6 specific pili. Scanning electron microscopy showed that the piliated bacteria attached to the leaf surface at the site of stomata. Non-piliated bacteria were evenly distributed on the leaf surface. All bacterial strains used in this study were capable of causing halo blight if injected into the plant. If the bacteria were sprayed on the plants, followed by spraying of sterile buffer, only piliated bacteria caused symptoms.

Function of pili in bacteriophage phi 6 penetration

The genome of bacteriophage phi 6, which has a lipid protein envelope, consists of three pieces of dsRNA. Virus infection is initiated by attachment to a phi 6-specific host pilus followed by fusion of the phage membrane and the bacterial outer membrane. In this study we analysed several different phi 6 hosts as well as more than 200 independently isolated phi 6-resistant variants derived from Pseudomonas syringae pv. phaseolicola. It is shown that phi 6-specific pili are coded by genes located in the host chromosome. It appears that pilus reaction is needed to pull the pilus-associated virus through the extracellular polysaccharide of the host and thus to bring it into contact with the outer membrane where membrane fusion can take place.

cDNA cloning of portions of the bacteriophage phi 6 genome

Phage phi 6 has a genome consisting of three pieces of double-stranded RNA. Single-stranded RNA was prepared from phi 6 nucleocapsids by in vitro transcription with the phage RNA polymerase. These transcripts were polyadenylated and used as templates for the preparation of cDNA copies. The resulting DNA was cloned into the PstI restriction nuclease site of plasmid pBR322. Insert-bearing plasmids were annealed to phi 6 RNA to assign the inserts to their proper segments. In this way we identified inserts corresponding to the large, medium, and small segments. Two large overlapping inserts of the small segment constitute the complete complement of the segment as determined by the sequence analysis of the DNA. In vitro coupled transcription and translation showed that the small segment inserts were able to direct the synthesis of the four known genes in the small segment. Two overlapping inserts in the medium segment constitute the entire segment and were shown to direct the in vitro synthesis of two of the three known proteins of the medium segment. Several inserts bearing about one-third the complement of the large segment were also isolated, and one of these directed the synthesis of a peptide that resembles protein P1. Restriction endonuclease maps were prepared for the inserts, and by in vitro synthesis it was possible to refine the genetic map of phi 6. A chimeric plasmid was constructed that combines plasmids pUC8 and RSF1010. Inserts placed on this plasmid were transformed to Pseudomonas phaseolicola, the natural host of phage phi 6. It was possible to refine further the genetic map by complementation of nonsense mutants of phi 6 with the cDNA.

phi 6-resistant phage-producing mutants of Pseudomonas phaseolicola

phi 6R mutants of Pseudomonas phaseolicola HB10Y were isolated and 12 out of 129 spontaneous mutants were found to produce phages during cellular growth. Following mitomycin C treatment 39 out of 82 isolated resistant mutants were phage-producing ones. The decrease in growth rate of these mutants corresponded roughly to the number of phages liberated into the medium. Prolonged storage of the mutants resulted in loss of production which could be regained by growing them with a high multiplicity of phages. The phage production phenomenon was independent of phage adsorption since both adsorbing and non-adsorbing phage-producing mutants were found. Occasionally, the phage-producing strains showed abnormally high numbers of intracellular phage particles in sectional material. The producers were found to be 2 to 5 times more resistant than the normal host or non-producing phi 6R strains to the lytic enzyme associated with the phi 6 virion, indicating that they had an altered cell wall structure. The lytic enzyme of phi 6, needed both in penetration and progeny release, was less active on the altered cell wall, leading to diminished infection efficiency and deficient plaque formation.