THE 6 GENES OF THE RUBISCO SMALL-SUBUNIT MULTIGENE FAMILY FROM MESEMBRYANTHEMUM-CRYSTALLINUM, A FACULTATIVE CAM PLANT

The nucleotide sequences of the entire gene family, comprising six gen es, that encodes the Rubisco small subunit (rbcS) multigene family in Mesembryanthemum crystallinum (common ice plant), were determined. Fiv e of the genes are arranged in a tandem array spanning 20 kb, while th e sixth gene is not closely linked to this array. The mature small sub unit coding regions are highly conserved and encode four distinct poly peptides of equal lengths with up to five amino acid differences disti nguishing individual genes. The transit peptide coding regions are mor e divergent in both amino acid sequence and length, encoding five dist inct peptide sequences that range from 55 to 61 amino acids in length. Each of the genes has two introns located at conserved sites within t he mature peptide-coding regions. The first introns are diverse in seq uence and length ranging from 122 bp to 1092 bp. Five of the six secon d introns are highly conserved in sequence and length. Two genes, rbcS -4 and rbcS-5, are identical at the nucleotide level starting from 121 bp upstream of the ATG initiation codon to 9 bp downstream of the sto p codon including the sequences of both introns, indicating recent gen e duplication and/or gene conversion. Functionally important regulator y elements identified in rbcS promoters of other species are absent fr om the upstream regions of all but one of the ice plant rbcS genes. Re lative expression levels were determined for the rbcS genes and indica te that they are differentially expressed in leaves.