Temperate phages are common and prophages are abundant residents of sequenced bacterial genomes. Mycobacteriophages are viruses infecting mycobacterial hosts including Mycobacterium tuberculosis and Mycobacterium smegmatis, encompass substantial genetic diversity, and are commonly temperate. Characterization of ten Cluster N temperate mycobacteriophages reveals at least five distinct prophage-expressed viral defense systems that interfere with infection of lytic and temperate phages that are either closely-related (homotypic defense) or unrelated (heterotypic defense). Target specificity is unpredictable, ranging from a single target phage to one-third of those tested. The defense systems include a single-subunit restriction system, a heterotypic exclusion system, and a predicted (p)ppGpp synthetase, which blocks lytic phage growth, promotes bacterial survival, and enables efficient lysogeny. The predicted (p)ppGpp synthetase coded by the Phrann prophage defends against phage Tweety infection, but Tweety codes for a tetrapeptide repeat protein, gp54, that acts as a highly effective counter-defense system. Prophage-mediated viral defense offers an efficient mechanism for bacterial success in host-virus dynamics, and counter-defense promotes phage co-evolution.

No abstract

The epidemiology and etiology of a disease of Fraxinus velutina (Arizona or velvet ash) called ash decline was investigated. Symptoms of ash decline include a progressive dieback of stems and branches as well as yellowing and necrosis of leaf tissue. The incidence and severity of ash decline in F. velutina cv. Modesto trees in the Phoenix, Arizona, U.S., metropolitan area was studied over an 8-year period. Symptom severity increased dramatically during this time period with a 30% mortality rate of trees in the study group. Etiological studies determined that phytoplasmas were associated with ash trees showing ash decline symptoms. Phytoplasmas were detected in the phloem sieve tube elements of plants using the DAPI microscopic test, and phytoplasma DNA was detected in ash tissue using a polymerase chain reaction (PCR) assay. The relationship of the phytoplasmas associated with ash decline to the ash yellows (AshY) phytoplasmas is discussed.

Ash decline (AD), caused by a phytoplasma, is a progressive dieback disease previously reported in Fraxinus velutina. The host range and distribution of AD is discussed based on samples collected from ash trees sampled from across the state of Arizona, U.S., including both wild-type F. velutina in riparian areas and F. velutina cv. Modesto, F. velutina cv. Rio Grande, and F. uhdei in landscaped areas. The results of the statewide survey indicate that AD is found in native and cultivated ash trees, with F. velutina cv. Modesto exhibiting the highest (100%) level of disease in the Phoenix metropolitan area. Wild-type F. velutina and F. velutina cv. Rio Grande also exhibited AD symptoms but with lower frequency and severity. Fraxinus uhdei did not exhibit symptoms of AD. Phytoplasma infection was detected in all tree types of F. velutina trees using DAPI staining and polymerase chain reaction (PCR) but was not detected in F. uhdei. Higher frequency of phytoplasma infection was detected in tree canopies versus roots in contrast to previous results reported in white ash. PCR was found to be more efficient at detecting the low-titer infection levels typical of F. velutina in comparison to DAPI staining.