I was doing a BLAST of the PsaA subunit of Photosystem I (PSI) restricted to Metazoans and found that the organism Trichoplax adhaerens had a fragment of 141 amino acids of this subunit. The predicted amino acid sequence from T. adhaerens has 97% similarity to that of the diatom Synedra acus.
T. adherens is a placozoan, a basal eumetazoan, in other words a very ancient animal.
It encompasses the region of PsaA from amino acid 615 to 755 using the numeration of the crystal structure from Thermosynechococcys elongatus 1jb0. This region contains the last two transmembrane helices of the PsaA subunit where some of the redox active chlorophylls are coordinated (see figure 1).
Figure 1. Cartoon model of the PsaA subunit from T. elongatus. In sand color the section of protein that is encoded in the PsaA fragment from T. adherens.
Because of the high similarity of the sequence to that of the diatom, and due to the fact that animals do not have photosystems, we can safely assume that it is an event of horizontal gene transfer... possibly too, contamination.
I will check if there are more fragments of photosynthetic proteins in this animal and report here if I find something else.
