MAP3Kα, a gene that encodes a key conserved protein kinase, is responsible for initiating a rapid cascade of cellular events leading to localized cell death. Hypersensitive response, as it is termed, enables genetically resistant plants to limit microbial invasion under the right environmental conditions. Since knowledge of close physically linked genes is important for genome analysis and possibly for improving disease resistance, systematic DNA sequence analysis, gene annotation, and protein BLASTs were performed to identify and characterize genes in close physical proximity to a MAP3Kα-like gene in Beta vulgaris L. US H20. On the same 125？Kb BAC, callose synthase (BvCS) and phytochrome A (PhyA) genes were within 50？Kb of MAP3Kα. The close physical linkage of these genes may result from selection for coordinated responses to disease pressure. Bert, a new chromodomain-carrying gypsy-like LTR retrotransposon, resides within an intron of the BvCS gene, where it is transcribed from the opposing strand. 1. Introduction A plant gene, MAP3Kα, produces a highly conserved protein product that activates hypersensitive response, a mechanism underlying R gene-mediated disease resistance [1]. In tobacco and in tomato, MAP3Kα activates cascades of enzymatic activations leading to a crescendo that is apoptosis or programmed cell death, a critical component of R gene-mediated disease resistance [1]. Research done on the crop plants, tomato and tobacco, as well as that performed on the model plant system Arabidopsis thaliana L. Heynth, over a 20-year period in a several laboratories, has presented adequate evidence that a particular gene called MAP3Kα is centrally important to R gene-mediated plant disease resistance [1]. In essence, a pathogen elicitor causes a conformational change in a plant protein initiating a cascade reaction leading to the so-called hypersensitive response, a primary countermeasure deployed by plants in order to effectively resist pathogen invasion. This key process is controlled by the protein product of MAP3Kα. In the genome of Arabidopsis thaliana, large-scale duplication of genetic regions followed by selective gene loss has created a recognized network of chromosomal synteny [2]. By developing physical genetic maps based on ESTs, Dominguez et al. [3] discovered conserved synteny with Arabidopsis among genomes of four phylogenetically divergent eudicot crops, namely, sugarbeet, potato, sunflower, and plum. In our previous study, complete BAC sequence analysis identified two core plant genes, CaMP and CKI, tightly physically linked to the
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