PLANT PHYSIOLOGY , Vol 107, Issue 3 985-994, Copyright © 1995 by American Society of Plant Biologists

GENE REGULATION AND MOLECULAR GENETICS

Nucleotide Sequence and Expression of the Genes for the [alpha] and [beta] Subunits of Phycocyanin in Cyanidium caldarium

R. F. Troxler, Y. Yan, J. W. Jiang and B. Liu
Department of Biochemistry, Boston University School of Medicine, 80 East Concord Street, Boston, Massachusetts 02118

The nucleotide sequence of the plastid-encoded operon containing genes for the [alpha] (cpcA) and [beta] (cpcB) subunits of phycocyanin in the unicellular red alga Cyanidium caldarium is described. cpcB is located 5[prime] to cpcA and the two genes are separated by a 102-bp spacer region. The transcription start site of cpcBA was mapped to 80 bp upstream of the ATG initiation codon of cpcB. Promoter-like elements similar to the -10 (TATAAT) and -35 (TTGACA) consensus promoters in bacteria were found 6 and 31 bp upstream of the transcription initiation site. Northern blotting revealed an abundant 1.3-kb cpcBA transcript in illuminated cells, but this transcript was undetectable in dark-grown cells. Expression levels of cpcBA in cells incubated with 10-6 M heme in the dark were similar to those in cells illuminated for 24 h. Cells illuminated with 150 [mu]M gabaculine (an inhibitor of [delta]-aminolevulinate synthesis) or 10 mM levulinic acid (an inhibitor of [delta]-aminolevulinate dehydrase) lacked detectable cpcBA transcripts. In cells illuminated with 200 [mu]M N-methyl-mesoporphyrin IX (an inhibitor of ferrocheletase), inhibition of cpcBA expression and phycocyanin synthesis was similar. These results provide strong evidence that light induction of the cpcBA operon is dependent on synthesis of heme.