A cryptic promoter in the exon of HKR1 drives expression of a truncated form of Hkr1 in Saccharomyces cerevisiae

Hansenula mrakii killer toxin resistant gene 1 (HKR1) is an intronless, single-exon gene that encodes Hkr1, the signaling mucin of the budding yeast Saccharomyces cerevisiae. HKR1 overexpression confers S. cerevisiae cells with resistance to the HM-1 killer toxin produced by the killer yeast Hansenula mrakii (currently known as Cyberlindnera mrakii). Hkr1 comprises multiple functional domains and participates in several signal transduction pathways, including the high-osmolarity glycerol (HOG) pathway, the cell wall integrity (CWI) mitogen-activated protein (MAP) kinase pathway, and the filamentation MAP kinase pathway; Hkr1 also controls bud-site selection. In this study, we identified a cryptic promoter in the HKR1 exon that regulates the transcription of a shorter transcript encoding a truncated form of Hkr1. This shorter protein still conferred resistance to the HM-1 killer toxin, suggesting that this cryptic promoter helps carry out Hkr1-mediated signal transduction efficiently by producing a specific Hkr1 domain with functions as a signaling messenger. Notably, reporter assays using the fluorescent protein gene mUkG1 and the β-galactosidase gene lacZ revealed that the transcriptional activity of this cryptic promoter was modulated by its upstream sequence within the single exon. Hkr1 thus differs from other signaling mucins, whose active C-terminal fragments are generated by post-translational processing, whereas the active C-terminal fragment of Hkr1 is generated by transcription from the exonic promoter. These findings describe a previously unknown example of functional diversification from a single gene, especially for a gene encoding a multidomain, multifunctional protein such as Hkr1.