However, expression of single-Trp-151 LacY with phenylalanine
in place of the remaining native tryptophan residues is poor.
Because phenylalanine residues are essentially randomly distributed
throughout transmembrane helical regions, whereas tyrosine
residues like tryptophan tend to be distributed near the
membrane–water interface (40), each tryptophan residue except
Trp-151 was replaced with tyrosine instead of phenylalanine,
which markedly increases expression with vector pCS19 (L.G.
and H.R.K., unpublished data).