Autoplaque formation in a Pseudomonas fluorescens strain: Phagelike particles and transactivation of the defective phage

Natural bacteriophages of Pseudomonas fluorescens are rare and its temperat e phages have not been described so far. In search for these phages, we hav e found that one of the P. fluorescens strains forms numerous small transpa rent autoplaques of different size and shape, which contained material repr oducible on the same strains. When centrifuged in a cesium chloride gradien t, this material yielded a band in the density zone of about 1.3 g/cm(3), w here protein components or bacteriophages with a relatively low content of nucleic acid are usually located. In the band material, electron microscopy revealed phagelike particles with empty and mostly undamaged heads and tai ls carrying in their distal region a formation resembling contracted sheath . DNA isolated from the preparation consisted of two components: a distinct 54-kb fragment, and a diffuse fragment ranging in size from 20 to 9.5 kb. Treatment of the large DNA fragment with various endonucleases yielded 42.2 - and 29.5-kb fragments (on average for different endonucleases); whereas t he same treatment of the diffuse fragment yielded two- to three distinct fr agments with the overall molecular sizes of 8.9 and 6.2 kb (for different n ucleases). We have suggested that cells harbor two different genetic elemen ts whose interaction results in the autoplaque appearance and in the format ion of negative colonies after infection with the autoplaque material. One of the two elements displays properties of a defective prophage with distur bed DNA synthesis and assembly, whereas the other exhibits the properties o f a transposable phage. After complementation or some other interaction bet ween these elements (transactivation, prophage induction caused by represso r inactivation), a bulk of defective phage particles devoid of DNA and a fe w DNA-containing particles were produced. It remains unclear whether both D NA types are contained in the same or different particles. The phage (or a system of elements) referred to as PT3 is noninducible. The phage mutants f orming larger negative colonies (NCs) were also revealed. Some of bacterial mutants resistant to PT3 infection produce the mutant phage with small and turbid NCs. PT3 produces no NCs on the lawns of other strains of the same or other pseudomonad species. This is the first case of describing a natura l temperate bacteriophage in P fluorescens. The two different elements of t his phage may represent the same genome of the defective prophage divided i nto two portions within a bacterial chromosome, each of which is capable of packaging into the phage head.