The pseudolysogen is not sensitive to φHSIC in plaque agar overlays, nor can additional prophages be induced by mitomycin C treatment ( 51). We have previously described the interaction of φHSIC with its host as a relatively unstable pseudolysogenic relationship, which yielded >10 11 phage/ml while enabling host growth to >2 × 10 9 cells/ml in broth cultures ( 51). Members of this taxon, formerly known as Vibrio pelagius, are fish pathogens that have been shown to produce tetrodotoxin ( 45) and piscicidal extracellular proteases ( 16) and to cause massive fish mortality in aquaculture facilities ( 48). We have previously isolated φHSIC and its host, a Listonella pelagia strain, from ocean waters near Oahu, Hawaii ( 25). Moebus ( 33) described a phage host system which simultaneously produced high phage titers and had unimpeded host cell growth, resulting in sigmoidal growth curves, attributed to partially resistant host cells. However, other definitions of pseudolysogeny have been put forward, such as a condition of starved cells, whereby phage-infected cells lack sufficient energy to lysogenize or have a lytic infection ( 42). In many ways, most marine environments behave like cultures of pseudolysogens in that they are combinations of sensitive and resistant hosts as well as temperate and virulent phages. Ackermann and DuBow attributed this to a mixture of either sensitive and resistant host cells or virulent and temperate phage. Preliminary evidence suggests that environmental factors may control the lysogenic switch (i.e., the decision between lytic and lysogenic gene expression) in marine microbial populations ( 23, 32, 52).īy the definition of Ackermann and DuBow ( 1), pseudolysogeny occurs when there is simultaneous occurrence of high numbers of phages and hosts in the same culture. Currently, lysogeny is detectable in oceanic populations only through prophage induction by the addition of DNA-damaging agents ( 38). Lysogeny has the potential to alter microbial phenotypes through conversion. Although viruses can modulate the abundance and diversity of microbial populations in the oceans through their lytic activity, their greatest role may be in silent viral infections resulting from lysogeny ( 24, 25, 36, 49, 50, 51, 52, 53). Undoubtedly the largest reservoir of viruses is the oceans, where they range in concentration from 10 4 to over 10 7/ml in the water column, and in excess of 10 8/cm 3 in the sediment ( 6, 40, 53). Phages are now recognized as the most abundant form of life on this planet, with their total numbers estimated at 10 31 in the biosphere ( 43). These results further emphasize the need to sequence phages from the marine environment, perhaps the largest reservoir of untapped genetic information. Because none of the phage-like ORFs were closely related to any existing phage sequences in GenBank (i.e., none more than 62% identical and most <25% identical at the amino acid level), φHSIC is unique among phages that have been sequenced to date. This evidence is consistent with a headful packaging mechanism similar to that of Salmonella phage P22 and Shigella phage Sf6. The genome was circularly permuted, with no physical ends detected by sequencing or restriction enzyme digestion analysis, and lacked a cos site. ![]() The genome (37,966 bp G+C content, 44%) contained 47 putative open reading frames (ORFs), 17 of which had significant BLASTP hits in GenBank, including a β subunit of DNA polymerase III, a helicase, a helicase-like subunit of a resolvasome complex, a terminase, a tail tape measure protein, several phage-like structural proteins, and 1 ORF that may assist in host pathogenicity (an ADP ribosyltransferase). φHSIC enters into a pseudolysogenic relationship with its host, Listonella pelagia, characterized by sigmoidal growth curves producing >10 9 cells/ml and >10 11 phage/ml. The genome for the marine pseudotemperate member of the Siphoviridae φHSIC has been sequenced using a combination of linker amplification library construction, restriction digest library construction, and primer walking.
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