Abstract
Although the paradigm for the acquisition of photosynthetic organelles is the endocytosis of cyanobacteria-like progenitors by heterotrophic protists, details of this evolutionary process are unclear. The small organellar chromosomes are remnants of the larger bacterial genomes with most genes from the endosymbiont's DNA having been either relocated to the protist's nucleus or entirely lost. As a result of those gene transfers, differences exist between plastids from different algal phyla and higher plants. We report here on the retention of a secY gene in cyanelle (= plastid) DNA of the eukaryotic protist Cyanophora paradoxa. This cyanelle secY encodes a functional protein homologous to SecY of Escherichia coli, identified as a subunit of the preprotein translocase complex. Similarity of the cyanelle and E. coli SecY topology, predicted from sequence information, has been confirmed experimentally through SecY-PhoA fusion protein analysis in E. coli. Cyanelle SecY, expressed in an E. coli secY mutant, substituted for the defective prokaryotic SecY. A plastid-encoded gene for a membrane protein functioning in protein transport across plastid membranes is unprecedented in higher plants. From these results we infer that a functional homolog of the prokaryotic preprotein translocation machinery is retained in some plastids.
Highlights
From the $Departmentof Biochemistry and 11Departments of Molecular and Cellular Biology and of Plant Sciences, the University of Arizona, Tucson, Arizona,85721 and the Illnstitut fur Allgemeine Biochemie und Ludwig-BoltzmannForschungsstelle fur Biochemie, Universitat Wien,A 1090 Wien
The paradigm for the acquisition of photosynthetic organelles is the endocytosis of cyanobacteria-like progenitors by heterotrophic protists, details of this evolutionary process are unclear
Plastids,like mitochondria, are known to possess a protein import machinery to take up a multitude of precursor polypeptides which are encoded by Cyanelles, photosynthetic organelles, are found in a few nuclear genes and are synthesized in the cytosol (11).It is to unicellular algae and are likely remnants of endosymbiotic be expected, that in addition to this protein import cyanobacteria with a small genome size,typical cyanobacterial apparatus other systems must exist by which intraorganellar phycobilisomes,inclusion bodies, and cyanobacterial pigment protein routing is achieved (12)
Summary
1.4 c and 23.9%, while among the latter identities rangefrom 25.6 to 41.1%. As expected, the overall identity scoreswere higher between plastid SecY proteins from C. paradoxa, the cryptomonad flagellateCryptomonas @ (52%)and chromophytic. For the remainder of the cy- sions” (Fig. 5b) These initial experimental data correspond anelle SecY protein, the predictedoverall orientation, as it is well with the observation that PhoA is enzymatically active drawn, correlates in the remaining 10 T M regions only when exported to the periplasm and cytoplasmic PhoA well with the established E. coli SecY topology (38) and is is inactive (18).Complementation analysis and theenzymatic supported by the FASTA alignment (Fig. 3). Membrane-spanning domains is consistent with th“epositive Complementation Analysis inan E. coli Mutant-As C. inside”rule (40), as arginyl and lysyl residuesoccurseven paradoxa is not transformable, cyanelle SecY was expressed times more frequently in stromal than in periplasmic loops.
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