Summary: Three new genes encoding the serine-aspartate (SD) repeat-containing proteins SdrC, SdrD and SdrE were found in strain Newman. The SD repeats had earlier been found in the fibrinogen-binding clumping factors ClfA and ClfB. The and genes encode high-molecular-mass fibrinogen-binding proteins that are anchored to the cell surface of . The genes now reported are closely linked and tandemly arrayed. The putative Sdr proteins have both organizational and sequence similarity to ClfA and ClfB. At the N-terminus, putative secretory signal sequences precede approximately 500 residue A regions. The A regions of the Sdr and Clf proteins exhibit only 20–30% residue identity when aligned with any other member of the family. The only conserved sequence is the consensus motif TYTFTDYVD. The Sdr proteins differ from ClfA and ClfB by having two to five additional 110–113 residue repeated sequences (-motifs) located between region A and the -region. Each -motif contains a consensus Ca-binding -hand loop normally found in eukaryotic proteins. The structural integrity of recombinant Sdr(B1-B5) protein comprising the five -repeats of SdrD was shown by bisANS fluorescence analysis to be Ca-dependent, suggesting that the -hands are functional. When Cawas removed the structure collapsed to an unfolded conformation. The original structure was restored by addition of Ca. The C-terminal -domains of the Sdr proteins contain 132–170 residues. These are followed by conserved wall-anchoring regions characteristic of many surface proteins of Gram-positive bacteria. The locus was present in all 31 strains from human and bovine sources tested by Southern hybridization, although in a few strains it contained two rather than three genes.


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