Polyvalency of erwiniaphage FE44 was characterized in accordance with the general concept of phage polyvalency. The phage host range as well as the mechanisms allowing of its expansion were determined. The ability to infect bacteria of different genera occupying fundamentally distinct ecological niches was shown to be a common feature of T7-like phages FE44, ВА14, T3 and T7. Phages of the T7-group are shown to be polyvalent. Bacteriophages FE44 and BA14 feature broad host ranges as they efficiently lyse three bacterial species of two different genera. They are able to reproduce in Erwinia "horticola", Erwinia amylovora and Escherichia coli cells. The coliphage T3 was shown to infect several E. "horticola"strains, as well. The mechanisms of host-driven phage exclusion and strategies of its escape by phages were estimated. A series of investigations of the phages' interaction with artificial single and double lysogens, as well as with natural plasmid-carrying and restriction-modification systems-carrying strains of E. "horticola" and E.coli were performed. Unlike other T7-like phages, bacteriophage FE44 reproduction is interfered by EcoB type I RM system. Nevertheless, in other cases phage FE44 acts similar to BA14, T3 and T7. All phages are abortively excluded in P1-lysogens by EcoP1I type III RM-system. Despite their polyvalence, the phages are as well excluded by resident prophage and plasmid elements regardless of the bacteria/phage used. In these cases, an exclusion occurs on the intracellular level due to a malfunction of phage progeny assembly. FE44 DNA was sequenced, annotated and deposited to GenBank (a.n. KF700371). Its genome of 39860 bp and GC content value of 48.62% reveals features typical for T7-like phages. All predicted genes (51 orf) are protein-coding sequences. A significant amount (78.43%) of homologous proteins is shared with phage T7. A set of conserved regulatory elements was identified. The phage was assigned to T7-like viruses genus (taxid 110456). A unique to phage FE44 (and most likely to BA14) determinant of host range expansion is defined by its receptor-binding protein (tail fiber protein, gp17) structure. FE44 tail fiber protein is elongated in comparison to phage T7 gp 17 protein. It is comprised of 666 aa as opposed to 553 aa in T7. The tail fiber elongation of the studied T7-like phages correlates with their host range expansion. Thus, the elongation of gp 17 protein in its middle region with the conservation of the distal domain structure allows a tail fiber to access a conservative receptor located in the core of LPS, which is stereochemichally hindered by distal parts of cell surface structures. Other determinants underlying the strategies of RM-systems of type I, II and III exclusion escape are common among T7-like phages and are regarded as group-specific. Phage populations of the isolates from quince and pear affected with fire blight disease were studied. Isolated bacteriophages are polyvalent as they were able to infect 3 species of 2 different genera of phytopathogenic bacteria. However, their host range does not include bacteria of distinct ecological niches. A phage of C1 morphotype, TT10-27 and a bacteriophage KEY of B1 morphotype, specific to E. amylovora, were identified. Therefore, this strain can be used to develop biological crop protection from agents of bacterial diseases. Keywords: segetal vegetation, agents of bacterial diseases, ecological niche, cereals.
