Figure 2

Genomic Context of the Thalassiosira petF Gene Transfer. The genomic context of the ferredoxin genes in T. oceanica and T. pseudonana indicates a recent gene transfer event. The T. pseudonana petF gene is part of the cp genome and is located between rpl19 and psaB(bottom left), serving as a model for the genomic situation in a common ancestor of the two species. The T. oceanica PETF gene is found in the nuclear genome next to a PDD gene encoding for a pyridoxal-dependent decarboxylase (upper right) with a conserved ortholog present in T. pseudonana (upper left). The sites of excision (chloroplast) and insertion (nuclear fragment) in T. oceanica show major genome rearrangements in conjunction with the presence of a novel protein coding gene orf127 in the chloroplast. Both nuclear and chloroplast sites in T. oceanica contain various forms of repeat elements („repeat", „74 bp IR") and motifs („C5") that point to the recombinational events underlying the complex multi-step gene transfer (upper and bottom right); these are not present at the comparable sites in T. pseudonana. An alignment between the T. pseudonana and T. oceanica ferredoxin genes reveals sequence similarities at what might be the 5' and 3' ends of the transferred DNA segment (center). The T. pseudonana petF coding sequence already contains a potential functional splice acceptor site near the N-terminus of the reading frame. A comparable situation in the ancestral T. oceanica cp genome would have facilitated the acquisition of an intron in a new genomic context leaving the major conserved part of the protein unaffected by the process (center left); the stem-loop (yellow bar), that serves as a transcriptional terminator for petF in the T. pseudonana cp genome, contains motifs that are retained in the nuclear genomic context of T. oceanica PETF, one of them being slightly modified to become an eukaryotic polyA-signal (center right).