A new function for the serine protease HtrA2 in controlling radiation-induced senescence in cancer cells.

Radiation therapy can induce cellular senescence in cancer cells, leading to short-term tumor growth arrest but increased long-term recurrence. To better understand the molecular mechanisms involved, we developed a model of radiation-induced senescence in cultured cancer cells. The irradiated cells exhibited a typical senescent phenotype, including upregulation of p53 and its main target, p21, followed by a sustained reduction in cellular proliferation, changes in cell size and cytoskeleton organization, and senescence-associated beta-galactosidase activity. Mass spectrometry-based proteomic profiling of the senescent cells indicated downregulation of proteins involved in cell cycle progression and DNA repair, and upregulation of proteins associated with malignancy. A functional siRNA screen using a cell death-related library identified mitochondrial serine protease HtrA2 as being necessary for sustained growth arrest of the senescent cells. In search of direct HtrA2 substrates following radiation, we determined that HtrA2 cleaves the intermediate filament protein vimentin, affecting its cytoplasmic organization. Ectopic expression of active cytosolic HtrA2 resulted in similar changes to vimentin filament assembly. Thus, HtrA2 is involved in the cytoskeletal reorganization that accompanies radiation-induced senescence and the continuous maintenance of proliferation arrest.

Host-dependent differences in abundance, composition and host range of cyanophages from the Red Sea.

Cyanobacteria coexist in the oceans with a wealth of phages that infect them. While numerous studies have investigated Synechococcus phages, much less data are available for Prochlorococcus phages. Furthermore, little is known about cyanophage composition. Here, we examined the abundance and relative composition of cyanophages on six cyanobacterial hosts in samples collected during spring and summer from the Red Sea. Maximal abundances found on Synechococcus of 35 000 phages/ml are within ranges found previously, whereas the 24 000 phages/ml found on Prochlorococcus are approximately 10-fold higher than previous findings. T7-like, T4-like and 'unknown' phages were isolated on all hosts, including many T4-like phages on high-light adapted Prochlorococcus strains, whereas TIM5-like phages were found only on Synechococcus. Large differences in cyanophage abundance and composition were found for different hosts on the same sampling date, as well as for the same host on different dates, with few predictable patterns discerned. Host range analyses showed that T7-like and TIM5-like phages were quite host-specific, whereas the breadth of hosts for T4-like phages was related to host type: those isolated on high-light adapted Prochlorococcus were considerably more host-specific than those on low-light adapted Prochlorococcus or Synechococcus. These host-related differences likely contribute to the complexity of host-phage interactions in the oceans.

Diversity and evolutionary relationships of T7-like podoviruses infecting marine cyanobacteria.

Phages are extremely abundant in the oceans, influencing the population dynamics, diversity and evolution of their hosts. Here we assessed the diversity and phylogenetic relationships among T7-like cyanophages using DNA polymerase (replication), major capsid (structural) and photosynthesis psbA (host-derived) genes from isolated phages. DNA polymerase and major capsid phylogeny divided them into two discrete clades with no evidence for gene exchange between clades. Clade A phages primarily infect Synechococcus while clade B phages infect either Synechococcus or Prochlorococcus. The major capsid gene of one of the phages from clade B carries a putative intron. Nearly all clade B phages encode psbA whereas clade A phages do not. This suggests an ancient separation between cyanophages from these two clades, with the acquisition or loss of psbA occurring around the time of their divergence. A mix and match of clustering patterns was found for the replication and structural genes within each major clade, even among phages infecting different host genera. This is suggestive of numerous gene exchanges within each major clade and indicates that core phage functions have not coevolved with specific hosts. In contrast, clustering of phage psbA broadly tracks that of the host genus. These findings suggest that T7-like cyanophages evolve through clade-limited gene exchanges and that different genes are subjected to vastly different selection pressures.