Ribosomal protein S4 is one of the proteins from the small ribosomal subunit. In Escherichia coli, S4 is known to bind directly to 16S ribosomal RNA. Mutations in S4 have been shown to increase translational error frequencies. It belongs to a family of ribosomal proteins which, on the basis of sequence similarities [1,2], groups:

• Eubacterial S4.
• Algal and plant chloroplast S4.
• Cyanelle S4.
• Archaebacterial S4.
• Mammalian S9.
• Yeast YS11 (SUP46).
• Marchantia polymorpha mitochondrial S4.
• Dictyostelium discoideum rp1024.
• Yeast protein NAM9 [3]. NAM9 has been characterized as a suppressor for ochre mutations in mitochondrial DNA. It could be a ribosomal protein that acts as a suppressor by decreasing translation accuracy.

S4 is a protein of 171 to 205 amino-acid residues (except for NAM9 which is much larger). The signature pattern for this protein is based on a conserved region located in the central section of these proteins.