A novel second streptomycete cyclophilin gene - designated - was isolated from a cosmid gene library of by using as gene probe a fragment of the previously isolated cyclophilin gene of the same organism. From its sequence the gene should encode a protein of 18868. Expression of in as a hexaHis-tagged fusion protein (H6ScCypB) and enzymic characterization of the purified protein showed that, like ScCypA, ScCypB is a peptidyl-prolyl isomerase (PPIase). The specific activity and substrate specificity of the enzyme were comparable to that of ScCypA, but it was threefold less sensitive to inhibition by cyclosporin A (CsA). In contrast to ScCypA, which is abundant and exists in free and liganded form, ScCypB was 50- to 100-fold less abundant in cytosol-derived protein fractions of or , as revealed by Western blot analyses, suggesting a specialized function for this enzyme in the streptomycete cell. Both and were found to be present as single copies in the genome of and hybridized to a single band in chromosomal DNAs of other streptomycetes. High-level expression of as well as of cloned into the expression vector pIJ702 did not produce detectable changes in growth and morphology of and Calculations of similarities to known cyclophilin sequences and construction of phylogenetic trees indicated that ScCypB and ScCypA are phylogenetically distant from each other. While ScCypA is clearly related to the eukaryotic cyclophilins, the analyses show the sequence of ScCypB to be the most divergent of all cyclophilin sequences, indicating that it possibly constitutes a cluster by itself.


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