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.

SummaryCaleosin is a Ca 2þ -binding oil-body surface protein. To assess its role in the degradation of oil-bodies, two independent insertion mutants lacking caleosin were studied. Both mutants demonstrated significant delay of breakdown of the 20:1 storage lipid at 48 and 60 h of germination. Additionally, although germination rates for seeds were not affected by the mutations, mutant seedlings grew more slowly than wild type when measured at 48 h of germination, a defect that was corrected with continued growth for 72 and 96 h in the light. After 48 h of germination, wild-type central vacuoles had smooth contours and demonstrated internalization of oil bodies and of membrane containing a-and d-tonoplast intrinsic proteins (TIPs), markers for protein storage vacuoles. In contrast, mutant central vacuoles had distorted limiting membranes displaying domains with clumps of the two TIPs, and they contained fewer oil bodies. Thus, during germination caleosin plays a role in the degradation of storage lipid in oil bodies. Its role involves both the normal modification of storage vacuole membrane and the interaction of oil bodies with vacuoles. The results indicate that interaction of oil bodies with vacuoles is one mechanism that contributes to the degradation of storage lipid.

The diplomonad parasite Giardia intestinalis contains two functionally equivalent nuclei that are inherited independently during mitosis. Although presumed to be asexual, Giardia has low levels of allelic heterozygosity, indicating that the two nuclear genomes may exchange genetic material. Fluorescence in situ hybridization performed with probes to an episomal plasmid suggests that plasmids are transferred between nuclei in the cyst, and transmission electron micrographs demonstrate fusion between cyst nuclei. Green fluorescent protein fusions of giardial homologs of meiosis-specific genes localized to the nuclei of cysts, but not the vegetative trophozoite. These data suggest that the fusion of nuclei, or karyogamy, and subsequently somatic homologous recombination facilitated by the meiosis gene homologs, occur in the giardial cyst.

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