Gram-negative bacteria produce a number of proteins which are secreted into the growth medium by a mechanism that does not require a cleaved N-terminal signal sequence. These proteins, while having different functions, seem [1] to share two properties: they bind calcium and they contain a variable number of tandem repeats consisting of a nine amino acid motif rich in glycine, aspartic acid and asparagine. It has been shown [2] that such a domain is involved in the binding of calcium ions in a parallel β roll structure. The proteins which are currently known to belong to this category are:

• Hemolysins from various species of bacteria. Bacterial hemolysins are exotoxins that attack blood cell membranes and cause cell rupture. The hemolysins which are known to contain such a domain are those from: E. coli (gene hlyA), A. pleuropneumoniae (gene appA), A. actinomycetemcomitans and P. haemolytica (leukotoxin) (gene lktA).
• Cyclolysin from Bordetella pertussis (gene cyaA). A multifunctional protein which is both an adenylate cyclase and a hemolysin.
• Extracellular zinc proteases: serralysin (EC 3.4.24.40) from Serratia, prtB and prtC from Erwinia chrysanthemi and aprA from Pseudomonas aeruginosa.
• Nodulation protein nodO from Rhizobium leguminosarum.

We derived a signature pattern from conserved positions in the sequence of the calcium-binding domain.