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cse, a Chimeric and Variable Gene, Encodes an Extracellular Protein Involved in Cellular Segregation in Streptococcus thermophilus

Laboratoire de Génétique et Microbiologie, UMR INRA 1128, IFR 110, Faculté des Sciences et Techniques de l'Université Henri Poincaré Nancy 1, Vandoeuvre-lès-Nancy, France,¹ Unité de Génétique, Institut des Sciences de la Vie, Université catholique de Louvain, Louvain-La-Neuve, Belgium²

Received 2 September 2004/ Accepted 4 January 2005

The isolation of a Streptococcus thermophilus CNRZ368 mutant displaying a long-chain phenotype allowed us to identify the cse gene (for cellular segregation). The N terminus of Cse exhibits high similarity to Streptococcus agalactiae surface immunogenic protein (SIP), while its C terminus exhibits high similarity to S. thermophilus PcsB. In CNRZ368, deletion of the entire cse open reading frame leads to drastic lengthening of cell chains and altered colony morphology. Complementation of the cse mutation with a wild-type allele restored both wild-type phenotypes. The central part of Cse is a repeat-rich region with low sequence complexity. Comparison of cse from CNRZ368 and LMG18311strains reveals high variability of this repeat-rich region. To assess the impact of this central region variability, the central region of LMG18311cse was exchanged with that of CNRZ368 cse. This replacement did not affect chain length, showing that divergence of the central part does not modify cell segregation activity of Cse. The structure of the cse locus suggests that the chimeric organization of cse results from insertion of a duplicated sequence deriving from the pcsB 3' end into an ancestral sip gene. Thus, the cse locus illustrates the module-shuffling mechanism of bacterial gene evolution.

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