Abstract
<title>Abstract</title> <p> The nitroplast of the haptophyte <italic>Braarudosphaera bigelowii</italic> is a nitrogen-fixing organelle derived from the cyanobacterium UCYN-A, and roughly half its proteins are host-encoded and imported via a targeting element termed the UCYN-A transit peptide (uTP). Published descriptions compare haptophyte uTPs with plant chloroplast transit peptides (cTP), though this separation may reflect proteome-wide amino-acid composition rather than a targeting grammar. We reanalysed a 740-entry import catalogue (735 with retrievable sequence) against 1,759 curated crop cTPs under eight controls holding species composition constant. The cross-species contrast reaches AUC 0.9814; a within-species contrast — C-terminal 120-residue windows against internal windows of the same proteins — gives 0.9425 ± 0.0167, and a redundancy- and background-corrected paired-delta contrast at 40% identity gives 0.9445; both round to 0.94, differing by one-eighth of a standard deviation. Restricted to the 76 mass-spectrometry-curated proteins free of profile-HMM circularity, the estimate is 0.8986 ± 0.0553. The same positional contrast across nine organisms spanning four clades — including <italic>Paulinella chromatophora</italic> , which imports host proteins into an independently derived primary endosymbiont — falls in a narrow null band of 0.5579–0.6274. Catalogue proteins carry a median 254-residue C-terminal extension relative to haptophyte orthologues that non-targeted controls lack. Four enriched tetrapeptides — IPRL and PDFP, previously reported, and RLLP and PRLL, reported here — survive Benjamini–Hochberg correction and homology filtering. The signature survives every control constructed; its published effect size is inflated, and recognition by an import machinery remains untested. </p>