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The Sinorhizobium meliloti ExpE1 protein secreted by a type I secretion system involving ExpD1 and ExpD2 is required for biosynthesis or secretion of the exopolysaccharide galactoglucan

Lehrstuhl für Genetik, Fakultät für Biologie, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany¹

Author for correspondence: Anke Becker. Tel: +49 521 106 5620. Fax: +49 521 106 5626. e-mail: anke.becker{at}genetik.uni-bielefeld.de

In Sinorhizobium meliloti the biosynthesis of the exopolysaccharide galactoglucan (EPS II) is directed by the exp genes. The expD1 and expD2 gene products are homologous to components of type I secretion systems. ExpE1, the gene of which is located adjacent to expD1 and expD2, was detected in S. meliloti cells and culture supernatants. ExpD1 and ExpD2 were required for the secretion of ExpE1, indicating that ExpE1 is secreted by a type I secretion system involving ExpD1 and ExpD2. ExpE1 contains 15 aspartate- and glycine-rich nonapeptide repeats that were suggested to bind Ca²⁺. The ability to bind Ca²⁺ was demonstrated for a recombinant ExpE1 protein. Extracellular EPS II was not detected in cultures of non-polar expD1, expD2 and expE1 deletion mutants implying that these three genes are required for biosynthesis or secretion of galactoglucan.

Keywords: Rhizobium meliloti, exopolysaccharides, type I secretion, Ca²⁺-binding protein

Abbreviations: ABC, ATP-binding cassette; EPS, exopolysaccharide; EPS I, succinoglycan; EPS II, galactoglucan; HMM, high molecular mass; LMM, low molecular mass; MBP, maltose-binding protein; MCS, multiple cloning site

^a Present address: Centro de Engenharia Biológica e Química, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.