The 283,757-bp double-stranded DNA genome of Pseudomonas fluorescens phage OBP shares a general genomic organization with Pseudomonas aeruginosa phage EL. Comparison of this genomic organization, assembled in syntenic genomic blocks interspersed with hyperplastic regions of the KZ-related phages, supports the proposed division in the "EL-like viruses," and the "phiKZ-like viruses" within a larger subfamily. Identification of putative early transcription promoters scattered throughout the hyperplastic regions explains several features of the KZ-related genome organization (existence of genomic islands) and evolution (multi-inversion in hyperplastic regions). When hidden Markov modeling was used, typical conserved core genes could be identified, including the portal protein, the injection needle, and two polypeptides with respective similarity to the 3=-5= exonuclease domain and the polymerase domain of the T4 DNA polymerase. While the N-terminal domains of the tail fiber module and peptidoglycan-degrading proteins are conserved, the observation of C-terminal catalytic domains typical for the different genera supports the further subdivision of the KZ-related phages. P seudomonas fluorescens phage OBP (vB_PflM_OBP) (30, 47) is member of a growing group of giant phages, isolated, to date, only from Pseudomonas species and represented by the completely sequenced and well-studied Pseudomonas aeruginosa myovirus KZ (9,18,19,[31][32][33]36,42). Three other phages encoding many proteins with similarity to KZ proteins have been completely sequenced: EL (23), 2012-1 (52), and PA3 (43). Krylov et al. (33) assigned 19 unsequenced Pseudomonas phages to this group. We refer to the group as KZ-related phages. Lavigne et al. (35) have argued that EL should be classified as a genus separate from KZ and 2012-1 based on its less extensive levels of similarity. By that criterion, PA3 belongs to the "phiKZ-like viruses" and the results reported here classify OBP as the second member of the possible genus EL-like viruses.OBP shares a number of definitive properties with the other KZ-related phages. Typically, KZ-related phages have a very large icosahedral head, ϳ122 nm in diameter, and a long (ϳ190-nm) contractile tail surrounded by fibers. A KZ-related phage head contains a proteinic inner body, which has been speculated to organize the packaged DNA (18, 32). The exceptionally large genomes of the KZ-related phages (between 211 and 317 kb of nonredundant sequence) are composed of circularly permuted and terminally redundant linear double-stranded DNAs as determined for KZ and 2012-1 (42, 52). The KZ-related genomes display a pronounced difference in GϩC content (between 36.8 and 48%) and all have markedly lower GϩC content than the chromosomes of their GC-rich Pseudomonas hosts (60 to 66% GϩC).This group of phages has numerous genes involved in nucleotide metabolism (e.g., thymidylate synthase, thymidylate kinase, ribonucleoside diphosphate reductase, subunit beta [NrdB], and dihydrofolate reductase) and at least six genes enco...