O ver millions of years of coevolution, retroviruses have developed strategies to overcome the tight connection between splicing and nuclear mRNA export to allow translation of introncontaining viral mRNAs and incorporation of full-length primary transcripts into particles in the cytoplasm. To achieve this, simple retroviruses rely on cis-acting sequence regions, known as constitutive export elements (CTEs), that depend entirely on factors encoded by the host cell genome. Complex retroviruses, on the other hand, encode accessory proteins that bind to intron-containing viral transcripts to facilitate their nucleocytoplasmic transport (1). These proteins include the Rev protein of HIV, the Rex protein of human T cell leukemia virus (HTLV), and the Rec protein of the human endogenous retrovirus HERV-K. These proteins bind selectively to intricate stem-loop structures in the terminal 3= region of retroviral transcripts, the so-called responsive elements (REs), and use the host protein Crm-1 to support nuclear exit through the pore complex. In addition to their regulatory roles in nuclear mRNA export, there is evidence that Rev and the other related retroviral proteins facilitate translation of the transported mRNAs by enhancing the association with polysomes and accelerating the encapsidation of viral transcripts (2-4).

Vectored vaccines based on highly attenuated modified vaccinia Ankara (MVA) are reported to be immunogenic, tolerant to pre-existing immunity, and able to accommodate and stably maintain very large transgenes. MVA is usually produced on primary chicken embryo fibroblasts, but production processes based on continuous cell lines emerge as increasingly robust and cost-effective alternatives. An isolate of a hitherto undescribed genotype was recovered by passage of a nonplaque-purified preparation of MVA in a continuous anatine suspension cell line (CR.pIX) in chemically defined medium. The novel isolate (MVA-CR19) replicated to higher infectious titers in the extracellular volume of suspension cultures and induced fewer syncytia in adherent cultures. We now extend previous studies with the investigation of the point mutations in structural genes of MVA-CR19 and describe an additional point mutation in a regulatory gene. We furthermore map and discuss an extensive rearrangement of the left telomer of MVA-CR19 that appears to have occurred by duplication of the right telomer. This event caused deletions and duplications of genes that may modulate immunologic properties of MVA-CR19 as a vaccine vector. Our characterizations also highlight the exceptional genetic stability of plaque-purified MVA: although the phenotype of MVA-CR19 appears to be advantageous for replication, we found that all genetic markers that differentiate wildtype and MVA-CR19 are stably maintained in passages of recombinant viruses based on either wildtype or MVA-CR.

Aerial parts of plants originate from pluripotent cells in the shoot apical meristem (SAM). This population of stem cells is maintained via a negative feedback loop involving stable expression of WUSCHEL (WUS) and CLAVATA3. SAM size is dynamic and undergoes a more than 2-fold expansion upon the transition to reproductive growth. The underlying mechanism controlling this process is largely unknown, but coinciding increased levels of trehalose 6-phosphate (T6P) suggest a participation of sugar signaling. Here we show that TREHALOSE PHOSPHATE PHOSPHATASE J, a component of the T6P pathway, is directly regulated by WUS, and controls SAM expansion at floral transition through WUS. Our findings demonstrate a dynamic feedback-regulation between central meristem maintenance and flowering time regulators with sugar signaling.

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