Abstract
The translocon at the outer envelope membrane of chloroplasts (Toc) mediates the recognition and initial import into the organelle of thousands of nucleus-encoded proteins. These proteins are translated in the cytosol as precursor proteins with cleavable amino-terminal targeting sequences called transit peptides. The majority of the known Toc components that mediate chloroplast protein import were originally identified in pea, and more recently have been studied most extensively in Arabidopsis. With the completion of the tomato genome sequencing project, it is now possible to identify putative homologues of the chloroplast import components in tomato. In the work reported here, the Toc GTPase cDNAs from tomato were identified, cloned and analyzed. The analysis revealed that there are four Toc159 homologues (slToc159-1, -2, -3 and -4) and two Toc34 homologues (slToc34-1 and -2) in tomato, and it was shown that tomato Toc159 and Toc34 homologues share high sequence similarity with the comparable import apparatus components from Arabidopsis and pea. Thus, tomato is a valid model for further study of this system. The expression level of Toc complex components was also investigated in different tissues during tomato development. The two tomato Toc34 homologues are expressed at higher levels in non-photosynthetic tissues, whereas, the expression of two tomato Toc159 homologues, slToc159-1 and slToc159-4, were higher in photosynthetic tissues, and the expression patterns of slToc159-2 was not significantly different in photosynthetic and non-photosynthetic tissues, and slToc159-3 expression was limited to a few select tissues.
Highlights
Plastids, whose evolutionary history can be traced to free-living cyanobacteria that were incorporated into a host cell through endosymbiosis, are organelles that perform essential metabolic and signaling processes in all plant cells [1,2]
Identification and Cloning of Tomato the outer envelope membrane of chloroplasts (Toc) GTPase cDNAs To identify putative Toc GTPase sequences from tomato, a BLASTn search was conducted against the Solanum lycopersicum genome sequences using the amino acid sequences of the Arabidopsis Toc GTPases as query sequences in 2010
The five sequences that were obtained from the tomato genome database with high homology to the Arabidopsis sequences were designated slToc34-1, slToc34-2, slToc159-1, slToc159-2 and slToc159-3
Summary
Plastids, whose evolutionary history can be traced to free-living cyanobacteria that were incorporated into a host cell through endosymbiosis, are organelles that perform essential metabolic and signaling processes in all plant cells [1,2]. There are many different types of plastids, all of which develop from proplastids and are interconvertible depending on the tissue in which they reside and the environmental conditions. The biogenesis and maintenance of specific plastid types in different tissues relies on the coordinated expression and import of thousands of nucleus-encoded proteins [3,4,5]; the vast majority of plastid proteins are encoded in the nucleus, as most of the genes from the original endosymbiont have been transferred to the nuclear genome during the evolutionary transition from free-living cyanobacteria to semi-autonomous organelle [6]. It is well established that import is facilitated by interactions between the intrinsic N-terminal transit peptide of the nuclear-encoded preproteins and a common recognition and translocation machinery located in the chloroplast double-membrane envelope [10]
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