Identification of Active-Site Histidine Residues of a Self-lncompatibility Ribonuclease from a Wild Tomato’

The style component of the self-incompatibility (S) locus of the wild tomato Lycopersicon peruvianum (L.) Mill. is an allelic series of glycoproteins with ribonuclease activity (S-RNases). Treatment of the S,-RNase from L . peruvianum with iodoacetate at p H 6.1 led to a loss of RNase activity. In the presence of a competitive inhibitor, guanosine 3”Ionophosphate (3‘-CMP), the rate of RNase inactivation by iodoacetate was reduced significantly. Analysis of the tryptic digestion products of the iodoacetate-modified S-RNase by reversed-phase high-performance liquid chromatography and electrospray-ionization mass spectrometry showed that histidine-32 was preferentially modified in the absence of 3’-GMP. Histidine-88 was also modified, but this occurred both in the presence and absence of 3’-CMP, suggesting that this residue is accessible when 3’-CMP is in the active site. Cysteine-150 was modified by iodoacetate in the absence of 3‘-CMP and, to a lesser extent, in its presence. The results are discussed with respect to the related fungal RNase T, family and the mechanism of S-RNase action.