Hypersensitive response (HR) in plants is generally characterized by a rapid and localized cell death, cell wall strengthening and synthesis of phytoalexins and PR proteins [1]. In order to understand the molecular and cellular defense mechanisms better during HR that results from the interaction between plant pathogen and its non-host, we inoculated hot pepper (Capsicum annuum cv. Pukang) leaves with soybean pustule pathogen, Xanthomonas campestris pv. glycines8ra. A pool of genes induced or repressed by infection of the pathogen has been isolated by using a di¡erential displaypolymerase chain reaction (DD-PCR) technique (Yi et al., unpublished data). Approximately one half of the isolated genes had no sequence similarity in existing databases and the major portion of identi¢ed DNA fragments consisted of enzymes involved in primary or secondary metabolic pathways. One of the fragments was found to have signi¢cant sequence homology with ATP:citrate lyase (ACL) known from diverse organisms including alga, fungi, and mammals.ACL is known to catalyze the following reaction: citrate+ CoA+ATPCoxaloacetate+ADP+Pi+acetyl-CoA. In the cytosol of animals, oleaginous yeast, and fungi, acetyl-CoA produced by the action of ACL is the major precursor for fatty acid and sterol biosynthesis [2]. However, in plants, the biosynthetic origin of cytosolic acetyl-CoA that is needed for biosynthesis of phytochemicals is not established. In sweet potato, ACL activity has been observed to increase 10-fold in root tissues response to infection by the fungal pathogen, Ceratocystis ¢mbriata, and to correspond to the accumulation of phytoalexin ipomeamarone [3], but the molecular characterization of a plant ACL has not yet been reported.In order to isolate a full-length cDNA clone encoding ACL, a cDNA library was constructed from hot pepper leaves inoculated with X. campestris pv. glycines8ra. After screening of the cDNA library by using a cDNA fragment of ACL produced by DD-PCR as a probe, a cDNA clone with an open reading frame containing 608 amino acids was ¢nally isolated and designated as Ca-ACL1 (GenBank accession number: AF290958). The deduced amino acid of Ca-ACL1 gene has signi¢cant sequence homology with the C-terminal part of known animal ACLs. Calculated molecular mass of hot pepper ACL polypeptide was 66 kDa, which is only half of the animal ACLs. The ACL catalytic center, GHAGA, and the histidine residue that is autophosphorylated by ATP during catalysis are highly conserved in Ca-ACL1, but none of the three additional phosphorylation sites (Tyr446, Ser450, Ser454) that are involved in the regulation of rat ACL activity exists [2].To investigate the expression kinetics of the Ca-ACL1 transcripts during inoculation with X. campestris pv. glycines8ra, total RNA was isolated from hot pepper leaves 0, 4, 8, and 12 h after inoculation, and Northern blot analysis was performed by using 32 P-labeled Ca-ACL1 as a probe. HR cell death on pepper leaves appeared at approximately 15 h post-in¢ltra-tion, whereas no visible symptom was de...