Rice Genetics IV - IRRI books - International Rice Research Institute

ments (Francastel et al 1999), are potential targets for reducing the incidence of genesilencing. Chromosomal location is important and the formation of protected localchromatin domains by the inclusion of matrix attachment region (MAR) elementsflanking the gene to be introduced has been proposed in order to minimize positioneffect (Kumpatla et al 1998, Mlynárová et al 1994, Spiker and Thompson 1996).Transgenic rice plants containing a uidA reporter construct flanked by MARs fromtobacco (Rb7) or yeast (ARS1) were reported to have significantly higher transgeneexpression levels than the controls, and GUS activity increased in proportion totransgene copy number up to 20 copies (Vain et al 1999). However, the data show thatthe constructs were extensively rearranged in the transgenic rice plants and the modesttwofold overall increase in expression may reflect the ability of AT-rich enhancerelements present in the MAR sequences to function as independent enhancers, ratherthan any chromatin effect (van der Geest and Hall 1997). Thus, additional experimentswith more discretely characterized transgene inserts and direct analysis of chromatindomain effects are needed to evaluate the contribution of MAR elements to thefidelity of transgene expression.Although sequence similarity is closely associated with methylation-based silencing,many endogenous genes are present in multiple copies. The small sequence differencespresent in such genes may be very important in avoiding detection by genomesurveillance processes and the introduction of promoter and coding sequencediversity between transgenes and resident genes has been proposed (Iyer et al 2000,Kumpatla et al 1998). These reviews also include consideration of other characteristicsof transgenes that make them visible to genome surveillance processes, such asthe disruption of normal gene functions and the possib ility that prokaryotic vectorsequences have signature sequences or structures that mark them as alien to the eukaryoticgenome (Bernardi 1995, Karlin et al 1998).Recently, the use of Arabidopsis as a model system has given remarkable insightinto gene assemblies that are involved in genome-silencing surveillance processes.Table 4 provides a synopsis of these genes. The mom gene (Amedeo et al 2000),which has yet to be cloned, is especially exciting in that it appears to be able to overcomePTGS for several genes without developmental penalties. It will be important todetermine whether expression of it, or its orthologs, can alleviate silencing in rice andother crop plants. Similarly, the som mutants (Elmayan et al 1998) appear to be effectiveagainst TGS in Arabidopsis and tests of their efficacy in crop plants are eagerlyawaited.476 Hall et al