Gene phylogenies and the endosymbiotic origin of plastids.

The endosymbiotic origin of chloroplasts from cyanobacteria has long been suspected and has been confirmed in recent years by many lines of evidence. Debate now is centered on whether plastids are derived from a single endosymbiotic event or from multiple events involving several photosynthetic prokaryotes and/or eukaryotes. Phylogenetic analysis was undertaken using the inferred amino acid sequences from the genes psbA, rbcL, rbcS, tufA and atpB and a published analysis (Douglas and Turner, 1991) of nucleotide sequences of small subunit (SSU) rRNA to examine the relationships among purple bacteria, cyanobacteria and the plastids of non-green algae (including rhodophytes, chromophytes, a cryptophyte and a glaucophyte), green algae, euglenoids and land plants. Relationships within and among groups are generally consistent among all the trees; for example, prochlorophytes cluster with cyanobacteria (and not with green plastids) in each of the trees and rhodophytes are ancestral to or the sister group of the chromophyte algae. One notable exception is that Euglenophytes are associated with the green plastid lineage in psbA, rbcL, rbcS and tufA trees and with the non-green plastid lineage in SSU rRNA trees. Analysis of psbA, tufA, atpB and SSU rRNA sequences suggests that only a single bacterial endosympbiotic event occurred leading to plastids in the various algal and plant lineages. In contrast, analysis of rbcL and rbcS sequences strongly suggests that plastids are polyphyletic in origin, with plastids being derived independently from both purple bacteria and cyanobacteria. A hypothesis consistent with these discordant trees is that a single bacterial endosymbiotic event occurred leading to all plastids, followed by the lateral transfer of the rbcLS operon from a purple bacterium to a rhodophyte.     

A mechanism for light-induced translation of the rbcL mRNA encoding the large subunit of ribulose-1,5-bisphosphate carboxylase in barley chloroplasts

Translational regulation plays a key role in light-induced expression of photosynthesis-related genes at various levels in chloroplasts. We here present the results suggesting a mechanism for light-induced translation of the rbcL mRNA encoding the large subunit (LS) of ribulose-1,5-bisphosphate carboxylase (Rubisco). When 8-day-old dark-grown barley seedlings that have low plastid translation activity were illuminated for 16 h, a dramatic increase in synthesis of large subunit of Rubisco and global activation of plastid protein synthesis occurred. While an increase in polysome-associated rbcL mRNA was observed upon illumination for 16 h, the abundance of translation initiation complexes bound to rbcL mRNA remained constant, indicating that translation elongation might be controlled during this dark-to-light transition. Toeprinting of soluble rbcL polysomes after in organello plastid translation showed that ribosomes of rbcL translation initiation complexes could read-out into elongating ribosomes in illuminated plastids whereas in dark-grown plastids, read-out of ribosomes of translation initiation complexes was inhibited. Moreover, new rounds of translation initiation could also occur in illuminated plastids, but not in dark-grown plastids. These results suggest that translation initiation complexes for rbcL are normally formed in the dark, but the transition step of translation initiation complexes entering the elongation phase of protein synthesis and/or the elongation step might be inhibited, and this inhibition seems to be released upon illumination. The release of such a translational block upon illumination may contribute to light-activated translation of the rbcL mRNA.