The AtCAO gene, encoding chlorophyll a oxygenase, is required for chlorophyll b synthesis in Arabidopsis thaliana

Chlorophyll b is synthesized from chlorophyll a and is found in the light-h arvesting complexes of prochlorophytes, green algae, and both nonvascular a nd vascular plants. We have used conserved motifs from the chlorophyll a ox ygenase (CAO) gene from Chlamydomonas reinhardtii to isolate a homologue fr om Arabidopsis thaliana. This gene, AtCAO, is mutated in both leaky and nul l chlorinal alleles, and DNA sequence changes cosegregate with the mutant p henotype. AtCAO mRNA levels are higher in three different mutants that have reduced levels of chlorophyll b, suggesting that plants that do not have s ufficient chlorophyll b up-regulate AtCAO gene expression. Additionally, At CAO mRNA levels decrease in plants that are grown under dimlight conditions . We have also found that the six major Lhcb proteins do not accumulate in the null ch1-3 allele.