The tendamistat gene can be used for the construction of fused genes with which fusion proteins are expressed and excreted in Streptomycetes host cells. The tendamistat portion can be modified, in particular it can be C-terminal shortened. When a gene for a shortened proinsulin in which the insulin B chain is linked to the A chain only via Lys or Lys-Lys is coupled to the tendamistat gene, this gene construction is introduced into an expression vector, and the latter is used to transform a Streptomycetes host cell, there is expression and secretion of the corresponding fusion protein. The fusion protein can easily be cleaved to give insulin precursors because of correctly established disulfide bonds. Genetic structures which code for the signal sequence and about the first ten amino acids of tendamistat as well as a desired protein are expressed in streptomyces host cells with a high yield, and the fusion proteins are secreted into the medium.