S1Fa was originally isolated from a spinach cDNA library as a cDNA encoding for a protein with specific binding activity for the tissue-specific negative cis-element S1F binding site of spinach rps1. The C-terminal contains a suggested NLS of the sequence (PPKKKK). The protein also contains a putative DNA-binding alpha helix (ISKKKMKRERLK). The mRNA accumulates in roots and etiolated seedlings more than in green leaves. The protein is well conserved across the proteomes of angiosperms, the authors posit this could describe a new class of plant transcription factors (PMID:7739894).