Abstract
The lacY genes from two Escherichia coli mutants, MAB20 and AA22, have been cloned in a multicopy plasmid by a novel 'sucrose marker exchange' method. Characterization showed that the plasmids express a lactose carrier with poor affinity for lactose. Neither mutant carried out concentrative uptake with methyl ß-d-galactopyranoside, lactose, or melibiose as the substrate. Nor did the mutants catalyze counterflow or exchange with methyl ß-d-galactopyranoside. Both mutants did, however, retain the capacity to carry out facilitated diffusion with lactose or melibiose. DNA sequencing revealed that MAB20 (histidine-322 to tyrosine) and AA22 (serine-306 to leucine) have amino acid substitutions within the putative 'charge-relay' domain thought to be responsible for proton transport. Galactoside-dependent H+ transport was readily measured in both mutants. We conclude, therefore, that the presence of a histidine residue at position 322 of the lactose carrier is not obligatory for H+ transport per se.
Original language | English (US) |
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Pages (from-to) | 253-264 |
Number of pages | 12 |
Journal | BBA - Biomembranes |
Volume | 982 |
Issue number | 2 |
DOIs | |
State | Published - Jul 10 1989 |
Externally published | Yes |
Keywords
- Cotransport
- Galactoside dependence
- Lactose carrier
- Proton transport
- Transport energetics
- lacY
ASJC Scopus subject areas
- Biophysics
- Biochemistry
- Cell Biology