Two proteins with different functions are derived from the KlHEM13 gene.

Two proteins that differ at the N terminus (l-KlCpo and s-KlCpo) are derived from KlHEM13, a single-copy-number gene in the haploid genome of Kluyveromyces lactis. Two transcriptional start site (tss) pools are detectable using primer extension, and their selection is heme dependent. One of these tss pools is located 5' of the first translation initiation codon (TIC) in the open reading frame of KlHEM13, while the other is located between the first and second TICs. In terms of functional significance, only s-KlCpo complements the heme deficiency caused by the Δhem13 deletion in K. lactis. Data obtained from immune detection in subcellular fractions, directed mutagenesis, chromatin immunoprecipitation (ChIP) assays, and the functional relevance of ΔKlhem13 deletion for KlHEM13 promoter activity suggest that l-KlCpo regulates KlHEM13 transcription. A hypothetical model of the evolutionary origins and coexistence of these two proteins in K. lactis is discussed.