A cDNA encoding the 57 kda subunit of a cytokinin-binding protein complex from tobacco: The subunit has high homology to s-adenosyl-l-homocysteine hydrolase

130 kDa cytokinin-binding protein complex (CBP130) is a major cytokinin binding entity in tobacco leaves (Nicotiana sylvestris) and contains two protein species of 57 and 36 kDa [CBP57 and CBP36, Mitsui and Sugiura (1993) Plant Cell Physiol. 34: 543]. We have determined partial amino acid sequences of CBP57 and isolated cDNAs encoding the protein from a tobacco cDNA library using an oligonucleotide probe. Sequence analysis revealed significant homology between CBP57 and S-adenosyl-L-homocysteine hydrolase from other organisms, which catalyzes the reversible hydrolysis of S-adenosyl-L-homocysteine, a methyltransferase inhibitor. CBP57 contains an additional sequence of 41 amino acids which is not present in animal and slime mold Sadenosyl-L-homocysteine hydrolases. This additional sequence is also found in the parsley and Rhodobacter enzymes, suggesting that it is unique to photosynthetic organisms. CBP57 is encoded by more than one nuclear genes in tobacco. Northern and western blot analyses revealed that the level of expression of the genes is high in roots and low in leaves. They are also expressed in cultured tobacco cells. We discuss the possibility that at least some of the physiological effects of cytokinin are mediated through the control of methylation/demethylation by regulating the intracellular concentration of S-adenosyl-L-homocysteine via the hydrolase.