Replication of bacterial plasmids in the nucleus of the red alga Porphyridium purpureum

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.