CRYPTIC TRANSPOSABLE PHAGES OF PSEUDOMONA S-AERUGINOSA

Frequencies of nucleotide sequences homologous to phage transposons (P T) of two species, D3112 and B3, were assessed in genomes of natural P seudomonas aeruginosa strains by the dot-blot hybridization method. Th ese strains were incapable of liberating viable phages on a lawn of th e PA01 standard indicator strain of P. aeruginosa. It was shown that t he homologies detected belong to two groups, high and intermediate, wi th respect to homology level. Homology patterns were classified as hig h when they provided signals comparable to those for hybridization in a positive control; they were classified as intermediate when the hybr idization level higher than the background level, but lower than in th e positive control. Homologous PT sequences were designated as cryptic PT. Intact cryptic PT prophages were shown to exist in genomes of par ticular natural strains manifesting a high level of hybridization. How ever, the growth of these phages was limited by the restriction system of strain PA01. It is possible to isolate strains maintaining the gro wth of a portion of cryptic PT. These strains differed from P. aerugin osa with respect to the specificity of the restriction and modificatio n system. Nevertheless, in most cases, the attempt to identify a novel host capable of maintaining growth of a cryptic PT failed. Natural st rains often carry cryptic PT related to both known PT species, D3112 a nd B3. The frequency of cryptic PT is extremely high, reaching 30% in only strains with a high level of homology and up to 50% in all strain s exhibiting homology. This high PT frequency is assumed to be associa ted with the considerable variation of P. aeruginosa.