Molecular analysis of the two maize chalcone synthase genes C2 and Whp (white pollen)

--Philipp Franken, Ursula Niesbach-Klösgen, Heinz Saedler and Udo Wienand

The C2 locus has been cloned from a Spm induced mutant c2-m1 using transposon tagging (Wienand et al., MGG 203:202, 1986). The analysis of a C2 specific cDNA confirmed that the locus is coding for a chalcone synthase (CHS) (Niesbach-Klösgen et al., J. Mol. Evol. 26:213, 1987). Genetic analysis indicates that C2 is not the only gene in maize encoding CHS. A second gene which can complement C2 function has been previously identified as the white pollen gene (Whp) (Coe et al., J. Hered. 72:318, 1981). This gene is of particular interest since the loss of CHS activity (in a double recessive mutant c2 c2 whp whp) leads to the production of sterile, white pollen.

Because of the predicted homology in function (and sequence) of both genes we used C2 specific cDNA as a molecular probe to clone the Whp gene. A homologous fragment segregating with the Whp locus could be identified from F2 progeny analysis of a Whp heterozygous population. This fragment was cloned, sequenced and compared to the genomic sequence of the C2 locus.

The two genes share high homology in the codogenic parts (94%) while the introns are very different in sequence and size (1524 bp for C2 and 2157 bp for Whp). The Whp clone isolated represents a functional gene since cDNA fragments homologous to the genomic clone have been isolated and sequenced. The putative amino acid sequence deduced from the genomic and cDNA sequences indicates that the two CHS proteins are very similar. Both proteins are 400 amino acids in length and differ in 21 amino acid positions. Sequences 5' and 3' of the codogenic sequences are significantly divergent indicating a different mode of regulation for the two genes.

Gene specific probes for both the C2 and the Whp gene have been isolated to analyze expression of both genes in various genotypes and different tissues. These analyses showed that Whp and C2 expression in aleurone is regulated differently.

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