Precursor of the transcription factor form (Processed sterol regulatory element-binding protein 1), which is embedded in the endoplasmic reticulum membrane (PubMed:15797383). Low oxygen or sterol conditions promote processing of this form, releasing the transcription factor form that translocates into the nucleus and activates transcription of genes required for adaptation to anaerobic growth (PubMed:15797383). Transcriptional activator required for transcription of genes required for adaptation to anaerobic growth like those implicated in the nonrespiratory oxygen-consumptive biosynthetic pathways of sterol, heme, sphingolipid, and ubiquinone biosynthesis. May monitor oxygen levels through sterol synthesis steps which require oxygen. Forms a tight complex with scp1, composed of 4 copies of scp1 and 4 copies of sre1, which protects sre1 precursor from degradation by the proteasome. Accumulates at promoters of the target genes (PubMed:16537923). Proteolytic processing to mature form is induced by low oxygen or sterols. The processed form binds to its own promoter to promote positive feedback regulation and ensure maximal activation. In low oxygen or sterol conditions sre1 undergoes proteolytic cleavage to release the soluble transcription factor from the membrane. Processed form is phosphorylated. Cells fail to grow in the absence of oxygen or sterols.