Genetic dissection of the co-expression of genes encoding the two isoforms of oleosins in the oil bodies of maize kernel.

Oleosins are abundant structural proteins on the surface of intracellular oil bodies in seeds. In many maize (Zea mays L.) inbreds, there are three oleosins, OLE18, OLE17, and OLE16, termed according to their apparent molecular weight, which are present in the proportional amounts of about 1:1:2 in isolated oil bodies. In some inbreds, OLE18 and OLE17 occur as molecular weight variants with a molecular weight difference of 1000 or less. In inbreds CM555 and FR2, OLE18 and OLE17, respectively, are absent; the respective genes ole18 and ole17 in the inbreds are present, but are transcriptionally inactive. The F-1 of CM555 x FR2 possesses both OLE18 and OLE17, as expected from the inheritance of ole18 and ole17 genes. In all inbreds examined, including CM555, FR2, and their F-1 hybrids, and in both the diploidic embryos and triploidic aleurone layers, the quantity of OLE18 and/or OLE17 equals that of OLE16. Since OLE18 and OLE17 are close members of the high-molecular weight (H) oleosin isoform whereas OLE16 belongs to the low-molecular weight (L) oleosin isoform, the results indicate the presence of equal amounts of the H and L isoforms in the oil bodies. The equality suggests the occurrence of a hetero-dimer, or -multimer, of the two isoforms. To probe the control mechanism for such a hetero-mer formation, CM555, FR2, and their F-1 hybrids, as well as Mo17 (a common inbred) have been used to analyze oleosin gene copy number, nuclear run-off transcription activity, transcript amount, and oleosin protein content. It has been determined that equal levels of H and L oleosin isoforms accumulate in oil bodies irrespective of the dosage of functional H and L genes. Therefore, the control of equality is not the consequence of the competition among the active H isoform genes for transcriptional machineries, but occurs at the post-transcriptional level, presumably at translation and/or through oleosin interaction (formation of heteromer) and stability.