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Structure of the ask-asd operon and formation of aspartokinase subunits in the cephamycin producer ‘Amycolatopsis lactamdurans’

Area de Microbiología, Facultad de Ciencias Biológicas y Ambientales, Universidad de León, 24071 León, Spain¹

Author for correspondence: Paloma Liras. Tel: +34 987 291504. Fax: +34 987 291506. e-mail: degplp{at}unileon.es

The first two genes of the lysine pathway are closely linked forming a transcriptional operon in the cephamycin producer ‘Amycolatopsis lactamdurans’. The asd gene, encoding the enzyme aspartic semialdehyde dehydrogenase, has been cloned by complementation of Escherichia coli asd mutants. It encodes a protein of 355 aa with a deduced M_r of 37109. The ask gene encoding the aspartokinase (Ask) is located upstream of the asd gene as shown by determination of Ask activity conferred to E. coli transformants. asd and ask are separated by 2 nt and are transcribed in a bicistronic 2·6 kb mRNA. As occurs in corynebacteria, the presence of a ribosome-binding site within the ask sequence suggests that this ORF encodes two overlapping proteins, Ask of 421 aa and M_r 44108, and Askß of 172 aa and M_r 18145. The formation of both subunits of Ask from a single gene (ask) was confirmed by using antibodies against the C-terminal end of Ask which is identical in both subunits. Ask activity of ‘A. lactamdurans’ is regulated by the concerted action of lysine plus threonine and this inhibition is abolished in E. coli transformants containing Ser³⁰¹ to Tyr, or Gly³⁴⁵ to Asp mutations of the ‘A. lactamdurans’ ask gene.

Keywords: aspartic semialdehyde dehydrogenase, beta-lactams

Abbreviations: -AAA, -aminoadipic acid; Asd, aspartate semialdehyde dehydrogenase; Ask, aspartokinase; DAP, diaminopimelate

The GenBank accession number for the sequence reported in this paper is AJ298904.

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