J. Gaffe et al., CHARACTERIZATION AND FUNCTIONAL EXPRESSION OF A UBIQUITOUSLY EXPRESSED TOMATO PECTIN METHYLESTERASE, Plant physiology, 114(4), 1997, pp. 1547-1556
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.