Flavobacterium psychrophilum is a serious pathogen in trout aquaculture, responsible for the diseases rainbow trout fry syndrome (RTFS) and cold water disease (CWD). Bacteriophage control of F. psychrophilum may constitute a realistic approach in the treatment of these diseases; however, a detailed understanding of the phage-host interactions is needed to evaluate the potential of F. psychrophilum bacteriophages for that purpose. Twenty-two F. psychrophilum phages from Danish rainbow trout farms were isolated and characterized. The phage genome sizes differed considerably and fell into three major size classes (8.5 to 12 kb, 48 kb, and 90 kb). The phage host ranges comprised from 5 to 23 of the 28 tested F. psychrophilum strains, and 18 of the phage isolates showed unique host ranges. Each bacterial strain had a unique pattern of susceptibility to the 22 phages, and individual strains also showed large variations (up to 10 7 -fold differences) in susceptibility to specific phages. Phage burst size (7 to 162 phages infected cell ؊1 ) and latency period (4 to 6 h) also showed pronounced differences both between phages and, for a specific phage, between host strains. In general, the characterization documented the presence of diverse F. psychrophilum phage communities in Danish trout farms, with highly variable patterns of infectivity. The discovery and characterization of broad-host-range phages with strong lytic potential against numerous pathogenic F. psychrophilum host strains thus provided the foundation for future exploration of the potential of phages in the treatment of RTFS and CWD.

The bacterial rRNA methyltransferase RlmA II (formerly TlrB) contributes to resistance against tylosinlike 16-membered ring macrolide antibiotics. RlmA II was originally discovered in the tylosin-producer Streptomyces fradiae, and members of this subclass of methyltransferases have subsequently been found in other Gram-positive bacteria, including Streptococcus pneumoniae. In all cases, RlmA II methylates 23S rRNA at nucleotide G748, which is situated in a stem±loop (hairpin 35) at the macrolide binding site of the ribosome. The conformation of hairpin 35 recognized by RlmA II is shown here by NMR spectroscopy to resemble the anticodon loop of tRNA. The loop folds independently of the rest of the 23S rRNA, and is stabilized by a non-canonical G±A pair and a U-turn motif, rendering G748 accessible. Binding of S.pneumoniae RlmA II induces changes in NMR signals at speci®c nucleotides that are involved in the methyltransferase±RNA interaction. The conformation of hairpin 35 that interacts with RlmA II is radically different from the structure this hairpin adopts within the 50S subunit. This indicates that the hairpin undergoes major structural rearrangement upon interaction with ribosomal proteins during 50S assembly.

A unique arrangement of promoter elements was found upstream of the bacteriophage P1 particle maturation gene (mat). A P1-specific late-promoter sequence with conserved elements located at positions ؊22 and ؊10 was expected from the function of the gene in phage morphogenesis. In addition to a late-promoter sequence, a ؊35 element and an operator sequence for the major repressor protein, C1, were found. The ؊35 and ؊10 elements constituted an active Escherichia coli 70 consensus promoter, which was converted into a P1-regulated early promoter by the superimposition of a C1 operator. This combination of early-and latepromoter elements regulates and fine-tunes the expression of the particle maturation gene. During lysogenic growth the gene is turned off by P1 immunity functions. Upon induction of lytic growth, the expression of mat starts simultaneously with the expression of other C1-regulated P1 early functions. However, while most of the latter functions are downregulated during late stages of lytic growth the expression of mat continues throughout the entire lytic growth cycle of bacteriophage P1. Thus, the maturation function has a head start on the structural components of the phage particle.The genomes of bacteriophages contain the genetic information to reprogram bacterial host cells to produce viral particles rather than new bacterial cells. A successful phage infection depends on the precisely regulated expression of this genetic information. Intuitively, viral DNA amplification should occur prior to viral particle formation and DNA packaging, which in turn should precede host cell lysis. Even slight deviations from this developmental program would have dramatic effects on infection efficiency and phage viability. Complex regulatory networks have been elucidated for phages like T4, , P2/P4, Mu, and T7, among others. The investigations of various regulatory phage proteins like antiterminators (9, 32), repressors (18, 35), activators (1, 19, 26, 36), sigma factors (8), antisigma factors (15), and RNA polymerases (27) provided major contributions to our understanding of principal regulatory concepts.For bacteriophage P1 only two major regulatory steps were described, early and late transcriptions (24). As a temperate phage, P1 has the ability to lysogenize its host (51). During lysogenic growth all lytic phage functions, many of which are toxic to the host, have to be silenced. To this end, P1 harbors a complex, tripartite immunity system with the major repressor protein, C1, as central regulator (for reviews on the P1 immunity system, see references 13 and 23). C1 is a DNA-binding repressor protein negatively regulating Escherichia coli consensus-like promoter sequences (12), which are superimposed by a C1 binding site (6, 41). Inactivation of C1 during lytic growth results in early transcription. Most P1 early functions are involved in phage DNA replication, but among them is also the late-promoter activator function gp10 (21, 24). gp10 in turn activates transcription from phage-specific late-promoter seq...

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