The zymogen of bacterial transglutaminase was found during cultivation of Streptoverticillium mobaraense (DSMZ strain) using rabbit antibodies raised against the active enzyme. Ion‐exchange chromatography at pH 5.0 yielded a highly purified pro‐enzyme. Structure information was obtained by means of Edman degradation and analysis of PCR amplified nucleotide fragments. The data revealed an excess of negatively charged amino acids in the pro‐region resulting in a decreased isoelectric point of the zymogen. Additionally, the new sequence gave rise to some modifications to the previously published hypothetical structure of prepro‐transglutaminase derived from genomic DNA [Washizu, K., Ando, K., Koikeda, S., Hirose, S., Matsuura, A., Takagi, H., Motoki, M. & Takeuchi, K. (1994) Biosci. Biotechnol. Biochem. 58, 82−87]. Inactive transglutaminase, which carries an activation peptide of 45 amino acids, has a calculated molecular mass of 42 445 Da. Its pro‐region provides for both suppression of activity and increased thermostability. Furthermore, it could be shown that the micro‐organism produces a protease which cleaves pro‐transglutaminase at the C‐side of Pro45. Rapid transformation of the mature enzyme also occurs by addition of other proteases. During conversion, 43 and 41 amino acid peptides are released by bovine trypsin and dispase from Bacillus polymyxa, respectively. The detection of endogenous substrates in the murein layer makes discussion of the physiological role of bacterial transglutaminases necessary.

Transglutaminases (TGases, EC 2.3.2.13) have proved to be valuable enzymes for site-directed protein coupling via N(epsilon)-(gamma-L-glutamyl)lysine bonds. Their use in conjugate synthesis would overcome many problems caused by chemical reagents. In this approach, we show for the first time that two proteins with different functionalities, namely soybean peroxidase and protein G, can be cross-linked by bacterial TGase with retention of their activities. Soybean peroxidase and protein G were chosen for the enzymic preparation of a bifunctional conjugate among a series of other TGase substrates detected by enzymic incorporation of small fluorescent or biotinylated molecules. The highest yields of conjugate were obtained with a 15-fold excess of peroxidase in phosphate buffer, pH 7.0. Size exclusion chromatography enabled both purification of the conjugates and recovery of the starting materials. Analysis of bifunctionality revealed the coupling of protein G with an average of three peroxidase molecules.