Although death is not a phenotype per se, the inviable mutants are informative nonetheless in so far as they demonstrate that the amino acids F 57 GIR 60 are absolutely required for viability. While F57 is universally conserved, it does not appear to be essential on its own for viability, as witnessed in the mild phenotypes of the F57A mutant. Similarly, all single amino acid changes explored here resulted in viable cells, suggesting a certain degree of biochemical/biophysical redundancy within this essential loop. In support of this notion, the strongest growth phenotypes observed across a range of temperatures and small molecule translational inhibitors were concentrated in the multiple alanine substitutions, i.e. 51-4A and 54-7A, thus directing the bulk of the biochemical and structural analyses to these two mutants.