A 2ND FORM OF ADENINE PHOSPHORIBOSYLTRANSFERASE IN ARABIDOPSIS-THALIANA WITH RELATIVE SPECIFICITY TOWARDS CYTOKININS

Adenine phosphoribosyltransferase (APRTase) is an important enzyme for its ability to convert adenine, a by-product of many biochemical reac tions, into AMP By functional complementation of an Escherichia coli m utant, cDNAs encoding two APRTases have been cloned from Arabidopsis t haliana. One of the cDNAs (ATapt1) has been previously identified whil e the second (ATap2) is of a previously unknown type. Kinetic analysis of the two enzymes purified from E. coli expressing the two cDNAs ind icates that ATapt2 has a higher affinity for cytokinin than the ATapt1 . RNase protection studies indicate that the ATapt2, is not expressed in leaves. Analysis of the gene structure indicates that ATapt2 has id entical intron positions to ATapt1, but neither the intron sequence no r intron size are conserved between the two genes. The implications of a second, differentially expressed APRTase with affinity for both ade nine and cytokinin are discussed.