Charakterisierung des Slr1649 aus Synechocystis sp. PCC 6803, ein Homolog zu Orf222 aus Guillardia theta

From all open reading frames (orfs), which are encoded on the nucleomorph of the cryptophyte Guillardia theta, 11 possess homologies to cyanobacterial orfs with unknown function. Beyond identifying the function of the protein, characterising such orfs may lead to a deeper understanding of the complex interactions of the four different genomes in Guillardia theta. The characterisation of the nucleomorph-encoded orf222 was accomplished by analysing a knock-out mutant of the gene’s homolog in Synechocystis sp. PCC 6803, i.e. slr1649. Slr1649 was demonstrated to be a putative phycocyanobilin lyase that specifically targets position Cys155 of the phycocyanin β-subunit. Findings from Shen et al. indicate that unlike several other phycobilin lyases, Slr1649 seems to function as a monomer or homo-multimer (Shen et al. 2006). Homologs of this putative lyase can be found in many cyanobacterial and some eukaryotic organisms. In cyanobacterial organisms, a correlation between the number of Slr1649 homologs and the number of phycobiliproteins (except allophycocyanin) could be detected. These homologs could be subdivided into the CpeT and Slr1649 groups. The members of the latter group are most likely phycocyanobilin lyases specific for position Cys155 of the phycocyanin β-subunit, as mentioned above. The members of the CpeT group, in which even Orf222 of Guillardia theta was able to be grouped, are putative phycoerythrobilin lyases specific for one conserved position at the phycoerythrin β-subunit. Although both groups are most likely functionally related, they exhibit differences in their genomic context. While the cpeT-group seems to possess a conserved position within or near the phycoerythrin operon, the slr1649-group shows no conservation in its genomic context. Besides Orf222, the transcript of a second putative phycoerythrobilin lyase, cpeZ, could be detected in the EST project of Guillardia theta. In contrast to Orf222, the CpeZ protein of Guillardia theta possesses a functional bipartite transit peptide which allows the translocation across the four plastid-surrounding membranes. Because CpeZ is probably part of the CpeZ/CpeY heterodimer, further analyses of the EST-library should reveal the transcript of cpeY.