Is plant biotin holocarboxylase synthetase a bifunctional enzyme?

Holocarboxylase synthetases (HCSs) catalyse the biotinylation of biotin-dep endent carboxylases in both prokaryotes and eukaryotes. In Escherichia coli and Bacillus subtilis, the protein also acts as a transcriptional represso r that regulates the synthesis of biotin. Previously, we isolated and chara cterized a cDNA encoding an Arabidopsis thaliana HCS and subsequently assig ned this enzyme form to the chloroplast compartment. To investigate whether or not the Arabidopsis protein may function as a regulator in E. coli, we have expressed the functional plant HCS in a birA-derepressed mutant strain of E. coli devoid of the corresponding E. coli protein and carrying a prom oter-less LacZ gene marker inserted into the biotin operon, such that the b io promoter drives the synthesis of beta-galactosidase. Our data demonstrat e th at although the expressed plant HCS efficiently complemented the funct ion of apo-carboxylase biotinylation in E. coli, it proved unable to regula te the expression of the biotin biosynthetic genes. (C) 2000 Academie des s ciences/Editions scientifiques et medicales Elsevier SAS.