Abstract
Rhodophytes (red algae) are a diverse group of algae with great ecological and economic importance. However, tools for post-genomic research on red algae are still largely lacking. Here, we report the development of an efficient genetic transformation system for the model rhodophyte Porphyridium purpureum. We show that transgenes can be expressed to unprecedented levels of up to 5% of the total soluble protein. Surprisingly, the transgenic DNA is maintained episomally, as extrachromosomal high-copy number plasmid. The bacterial replication origin confers replication in the algal nucleus, thus providing an intriguing example of a prokaryotic replication origin functioning in a eukaryotic system. The extended presence of bacterial episomal elements may provide an evolutionary explanation for the frequent natural occurrence of extrachromosomal plasmids in red algae, and may also have contributed to the high rate of horizontal gene transfer from bacteria to the nuclear genome of Porphyridium purpureum and other rhodophytes.
Highlights
Rhodophytes are a diverse group of algae with great ecological and economic importance
To identify selectable marker genes that are potentially suitable for the development of a genetic transformation system for P. purpureum, the sensitivity of algal cells to possible selection agents was determined
Despite the enormous ecological and economic importance of rhodophytes, molecular research on red algae is lagging behind research on plants and green algae
Summary
Rhodophytes (red algae) are a diverse group of algae with great ecological and economic importance. The extended presence of bacterial episomal elements may provide an evolutionary explanation for the frequent natural occurrence of extrachromosomal plasmids in red algae, and may have contributed to the high rate of horizontal gene transfer from bacteria to the nuclear genome of Porphyridium purpureum and other rhodophytes. A few red algae are on the way to becoming model organisms, including the thermoacidophilic species Cyanidioschyzon merolae and Galdieria sulphuraria, the red seaweed Chondrus crispus, and the mesophilic rhodophyte Porphyridium purpureum Their genomes have been sequenced and some tools for molecular research in these species have been developed, most notably a genetic transformation system for Cyanidioschyzon merolae that seems to exhibit a relatively high rate of homologous recombination[3]. Why P. purpureum is such an efficient recipient of genes that are horizontally transferred from bacteria is currently unknown
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