TABLE 3.

Expression levels of LTB mutants in ETEC

MutationCodon changeaBlood sugar binding pocket locationfExpression level in ETECb
Q3KCAG→AAAInWild type
Q3LCAG→CTGInPoor
Y18ATAT→GCCInPoor
K23AAAG→GCGOutOnly LTA detected
L25ECTA→GAAOutWild type
L25ICTA→ATCOutPoor
F42ETTT→GAGOutNone detected
F42YTTT→TATOutNone detected
K43RAAG→CGTOutWild typec
A46DGCA→GACInReduced
A46SGCA→AGCInReduced
T47AACA→GCAInWild type
F48YTTT→TATOutNone detected
Q49LCAG→CTGOutWild typec
T80AdACC→GCCOutWild typec
D83LGAT→CTGOutPoor
N94LeAAT→CTGInWild typec
S100KAGT→AAAOutPoor
S100LAGT→CTGOutPoor
  • a Oligonucleotides used to generate these mutants are listed in Table 2 (see Table S1 in the supplemental material).

  • b LTB mutants were transformed into jf570, and expression level was examined by immunoblot analysis of CFU-matched total culture samples using cross-reactive anti-CT antibody.

  • c Mutant also exhibited wild-type behavior in preliminary cell surface binding assays.

  • d This mutant was generated using XL-1 Red mutator cells (Stratagene), according to the manufacturer's instructions; it also has a silent mutation at Tyr-15 (ACA→ACT).

  • e This residue's interaction with blood sugar residues is only through its backbone.

  • f In, inside; Out, outside.