The nature of the ssyF29 mutation causing the synthesis of a truncated form of the ribosomal protein S1 and its location in the rpsA gene were determined. The ssyF mutation was found to result from insertion of the IS10R element which causes the termination of translation of the corresponding mRNA at the first insertion nucleotide and the production of the S1 protein which is truncated at the C-terminus and composed of 464 amino acid residues (instead of 557 residues in the wild-type protein). The mutant rpsA gene (ssyF) encodes no additional amino acid residues as compared with the wild-type rpsA gene. © 1996 MAEe Cyrillic signK Hayκa/Interperiodica Publishing.