The bchI gene (synonym: chlI), whose product is involved in magnesium chelatase activity, is located in the small single-copy region of the plastid genome of Heterosigma carterae (heterokont alga, Raphidophyceae; formerly named Olisthodiscus luteus). As a unique feature, bchI in H. carterae overlaps an upstream open reading frame (ORF97) of unknown function by 23 base-pairs. Cells contain mRNAs for the full (1.6 kb) cotranscript as well as transcripts for bchI (1.2 kb) and ORF97 (0.4 kb). Transcription initiation in a plastid run on assay of the ORF97/bchI gene cluster is approximately 5-fold higher in cultures sampled in the light versus the dark. In contrast, Northern analysis shows that all three transcripts are in equal abundance at both light and dark sampling times. An antibody raised against a BchI fragment expressed in Escherichia coli recognized a protein of the expected size in the plastid fraction of H. carterae on a Western blot. Analysis of BchI amino acid sequences suggests that the protein may be membrane translocated and could bind ATP. Phylogenetic analysis of bchI sequences documents a deep evolutionary branching between chlorophyll a/b and non-chlorophyll b plastids.