THE Y1 GENE OF MAIZE CODES FOR PHYTOENE SYNTHASE

The cloned y1 locus of maize was sequenced and found to encode phytoen e synthase. Different ''wild-type'' alleles of the locus were found to differ by the insertion of transposable elements in their promoter an d polyA addition regions, and by the length of a CCA tandem repeat ser ies, without any obvious effect on function of the gene. A dominant Y1 (''wild-type'') allele was observed to be expressed at highest levels in the seedling but also in the embryo and endosperm. The Mu3 transpo sable element insertion responsible for a pastel allele of y1, which g ives lowered levels of carotenoids in the endosperm of kernels and see dlings grown at high temperatures, was located in the 5' end of the ge ne. Although the size of the transcript from this y1 mutation suggests that the Mu3 element provides the promoter for this allele, leaf tiss ue in this mutant line contained approximately normal amounts of y1 mR NA. A recessive allele of y1, which conditions normal levels of carote noids in the embryo and seedling, but almost no carotenoids in the end osperm, was found to accumulate normal amounts of y1 mRNA in the seedl ing and embryo, while y1 transcripts were not detected in the endosper m.