Two differentially regulated mRNAs with different 5' ends encode secreted with intracellular forms of yeast invertase

The SUC2 gene of yeast (Saccharomyces) encodes two forms of invertase: a secreted, glycosylated form, the synthesis of which is regulated by glucose repression, and an intracellular, nonglycosylated enzyme that is produced constitutively. The SUC2 gene has been cloned and shown to encode two RNAs (1.8 and 1.9 kb) that differ at their 5' ends. The stable level of the larger RNA is regulated by glucose; the level of the smaller RNA is not. A correspondence between the presence of the 1.9 kb RNA and the secreted invertase, and between the 1.8 kb RNA and the intracellular invertase, was observed in glucose-repressed and -derepressed wild-type cells. In addition, cells carrying a mutation at the SNF1 locus fail to derepress synthesis of the secreted invertase and also fail to produce stable 1.9 kb RNA during growth in low glucose. Glucose regulation of invertase synthesis thus is exerted, at least in part, at the RNA level. A naturally silent allele (suc2 degrees) of the SUC2 locus that does not direct the synthesis of active invertase was found to produce both the 1.8 and 1.9 kb RNAs under normal regulation by glucose. A model is proposed to account for the synthesis and regulation of the two forms of invertase: the larger, regulated mRNA contains the initiation codon for the signal sequence required for synthesis of the secreted, glycosylated form of invertase; the smaller, constitutively transcribed mRNA begins within the coding region of the signal sequence, resulting in synthesis of the intracellular enzyme.