Tracking the early events of photosymbiosis evolution.

Oxygenic photosynthesis evolved in cyanobacteria around 3.2 giga-annum (Ga) ago and was acquired by eukaryotes starting around 1.8 Ga ago by endosymbiosis. Photosymbiosis results either from integration of a photosynthetic bacteria by heterotrophic eukaryotes (primary photosymbiosis) or by successive integration of photosymbiotic eukaryotes by heterotrophic eukaryotes (secondary photosymbiosis). Primary endosymbiosis is thought to have been a rare event, whereas secondary and higher-order photosymbiosis evolved multiple times independently in different taxa. Despite its recurrent evolution, the molecular and cellular mechanisms underlying photosymbiosis are unknown. In this opinion, we discuss the primary events leading to the establishment of photosymbiosis, and we present recent research suggesting that, in some cases, domestication occurred instead of symbiosis, and how oxygen and host immunity can be involved in symbiont maintenance.

Reptin and Pontin function antagonistically with PcG and TrxG complexes to mediate Hox gene control.

Pontin (Pont) and Reptin (Rept) are paralogous ATPases that are evolutionarily conserved from yeast to human. They are recruited in multiprotein complexes that function in various aspects of DNA metabolism. They are essential for viability and have antagonistic roles in tissue growth, cell signalling and regulation of the tumour metastasis suppressor gene, KAI1, indicating that the balance of Pont and Rept regulates epigenetic programmes critical for development and cancer progression. Here, we describe Pont and Rept as antagonistic mediators of Drosophila Hox gene transcription, functioning with Polycomb group (PcG) and Trithorax group proteins to maintain correct patterns of expression. We show that Rept is a component of the PRC1 PcG complex, whereas Pont purifies with the Brahma complex. Furthermore, the enzymatic functions of Rept and Pont are indispensable for maintaining Hox gene expression states, highlighting the importance of these two antagonistic factors in transcriptional output.

Myc interacts genetically with Tip48/Reptin and Tip49/Pontin to control growth and proliferation during Drosophila development.

The transcription factor dMyc is the sole Drosophila ortholog of the vertebrate c-myc protooncogenes and a central regulator of growth and cell-cycle progression during normal development. We have investigated the molecular basis of dMyc function by analyzing its interaction with the putative transcriptional cofactors Tip48/Reptin (Rept) and Tip49/Pontin (Pont). We demonstrate that Rept and Pont have conserved their ability to bind to Myc during evolution. All three proteins are required for tissue growth in vivo, because mitotic clones mutant for either dmyc, pont,or rept suffer from cell competition. Most importantly, pont shows a strong dominant genetic interaction with dmyc that is manifested in the duration of development, rates of survival and size of the adult animal and, in particular, of the eye. The molecular basis for these effects may be found in the repression of certain target genes, such as mfas, by dMyc:Pont complexes. These findings indicate that dMyc:Pont complexes play an essential role in the control of cellular growth and proliferation during normal development.

Analysis of paralogous pontin and reptin gene expression during mouse development.

Evolutionarily conserved from yeast to human, the paralogous DNA helicases Pontin (Pont) and Reptin (Rept) are simultaneously recruited in multi-protein chromatin complexes that function in different aspects of DNA metabolism (transcription, replication and repair). When assayed, the two proteins were found to be essential for viability and to play antagonistic roles, suggesting that the balance of Pont/Rept regulates epigenetic programmes critical for development. Consistent with this, the two helicases are provided in the same embryonic territories during Drosophila development. In Xenopus, while transcribed in the same regions early in embryogenesis, pont and rept adopt significantly different patterns afterwards. Here we report that the two genes follow highly resembling transcription patterns in mouse embryos, with prominent expression in limb buds and branchial arches, organs undergoing mesenchymal-epithelial interactions and in motoneurones from cranial and spinal regions. Thus, simultaneous expression during development appears to constitute another feature of the evolutionary conservation of pont and rept genes.