Evolution of ribosomal protein s14 demonstrated by the reconstruction of chimeric ribosomes in bacillus subtilis

Ribosomal protein S14 can be classified into three types. The first, the C1 type has a Zn21 binding motif and is ancestral. The second and third are the C2 short and C2 long types, neither of which contain a Zn21 binding motif and which are ca. 90 residues and 100 residues in length, respectively. In the present study, the C1 type S14 from Bacillus subtilis ribosomes (S14BsC1) were completely replaced by the heterologous C2 long type of S14 from Escherichia coli (S14Ec) or Synechococcus elongatus (S14Se). Surprisingly, S14Ec and S14Se were incorporated fully into 70S ribosomes in B. subtilis. However, the growth rates as well as the sporulation efficiency of the mutants harboring heterologous S14 were significantly decreased. In these mutants, the polysome fraction was decreased and the 30S and 50S subunits accumulated unusually, indicating that cellular translational activity of these mutants was decreased. In vitro analysis showed a reduction in the translational activity of the 70S ribosome fraction purified from these mutants. The abundance of ribosomal proteins S2 and S3 in the 30S fraction in these mutants was reduced while that of S14 was not significantly decreased. It seems likely that binding of heterologous S14 changes the structure of the 30S subunit, which causes a decrease in the assembly efficiency of S2 and S3, which are located near the binding site of S14. Moreover, we found that S3 from S. elongatus cannot function in B. subtilis unless S14Se is present.