Transglutaminase (TGase)is a multifunctional enzyme vital for many physiologic processes, such as cell differentiation, tissue regeneration, and plant pathogenicity. The acyl transfer function of the enzyme can activate primary amines and, consequently, attach them onto a peptidyl glutamine, a reaction important for various in vivo and in vitro protein crosslinking and modification processes. To understand better the structure-function relationship of the enzyme and to develop it further as an industrial biocatalyst, we studied TGase secreted by several Streptomyces species and Phytophthora cactorum We purified the enzyme from S. lydicus S. platensis S. nigrescens S. cinnamoneus and S. hachijoensis The pH and temperature profiles of S. lydicus S. platensis and S. nigrescens TGases were determined. The specificity of S. lydicus TGase toward its acylaccepting amine substrates was characterized. Correlation of the electronic and steric features of the substrates with their reactivity supported the mechanism previously proposed for Streptomyces mobaraensis TGase.