CHARACTERIZATION AND FUNCTIONAL EXPRESSION OF A UBIQUITOUSLY EXPRESSED TOMATO PECTIN METHYLESTERASE

Pectin methylesterase (PME), a ubiquitous enzyme in plants, de-esterif ies the methoxylated pectin in the plant cell wall. We have characteri zed a PME gene (designated as pmeu1) from tomato (Lycopersicon esculen tum) with an expression that is higher in younger root, leaf, and frui t tissues than in older tissues. Hypocotyls and epicotyls show higher accumulation of pmeu1 transcripts compared with cotyledons. pmeu1 repr esents a single-copy gene in the tomato genome. Comparison of the dedu ced amino acid sequence of pmeu1 with other PME homologs showed that t he N-terminal halves are highly variable, and the C-terminal halves ar e relatively conserved in plant PMEs. Constitutive expression of a fru it-specific PME antisense gene does not affect the level of pmeu1 tran scripts in vegetative tissues but does lower the level of PMEU1 mRNA i n developing tomato fruits. These results suggest that there exists de velopmentally regulated silencing of pmeu1 by a heterologous PME antis ense gene. Expression of pmeu1 in tobacco (Nicotiana tabacum) under th e control of the cauliflower mosaic virus 35S promoter caused up to a 4-fold increase in PME specific activity that was correlated with the accumulation of PMEU1 mRNA. In vitro transcription-translation analyse s show that pmeu1 encodes a 64-kD polypeptide, whereas transgenic toba cco plants expressing pmeu1 accumulate a new 37-kD polypeptide, sugges ting extensive posttranslational processing of PMEU1. These results ar e the first evidence, to our knowledge, of the functional characteriza tion of a PME gene and the extensive modification of the encoded polyp eptide.