A number of clones have been isolated from two species which complement the PhoA phenotype of mutants under conditions that induce the expression of alkaline phosphatase (APase). These clones were initially thought to carry XPases because the transformed host could hydrolyse a common APase substrate, XP (5-bromo-4-chloro-3-indolyl-phosphate). The sequences of the open reading frames responsible for the phenotypic complementation showed no sequence similarity to APases of , human (bone-liver-kidney, intestinal or placental) or Therefore, these clones were designated as XPA (for X Phosphatase Activity) clones. Four of the clones encoded small (10 kDa), basic, hydrophobic proteins. Two of these, from 168 and from MC14, shared 62% identity at both the DNA and the predicted amino acid sequence level. The fact that homologues from two strains were cloned indicated that the screen was specific, but not for APase genes. It is clear that phenotypic complementation with cloned DNA from another genus does not ensure the identification of an APase gene. Possible mechanisms for the abnormal phenotypic complementation are discussed.


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