Title

Differential expression and substrate specificities of sugar binding proteins in Thermotoga maritima

Date of Completion

January 2005

Keywords

Biology, Molecular|Biology, Microbiology

Degree

Ph.D.

Abstract

In this study, high-affinity maltose- and glucose-binding activities in cell-free extracts of Thermotoga maritima were detected; these activities were distinct and specific. In periplasm extracts of cells grown on maltose, low maltose-binding activity was present compared to glucose-binding activity. While in periplasmic extracts of cells grown on glucose lower glucose binding activity was present compared to maltose binding activities Annotation of the complete genome sequence of T. maritima suggests that there may be at least two maltose binding proteins, MalE1 and MalE2, encoded in its genome. The protein components comprising these paralogous maltose ABC transporters show high sequence similarity to one other. Phylogenetic analyses of the MalE, MalF and MalG homologs of T. maritima showed these all group with their respective homologs from enteric bacteria, Thermobifida fusca and Corynebacterium efficiens YS-314 suggesting horizontal transfer of these maltose transporter genes among these bacteria. In contrast to MalEFG, the MalK homologs from T. maritma and C. efficiens are related to E. coli MalK. The substrate specificities and affinities of substrate binding proteins MalE1 and MalE2 were measured and found to differ and are also expressed under different growth conditions. MalE1 binds maltose (Kd 24 ± 1 μM), maltotriose (Kd 8 ± 0.5 nM), and β-(1 → 4)-mannotetraose (Kd 38 ± 1 μM). MalE2 binds maltose (Kd 8.4 ± 1 μM), maltotriose (Kd 11.5 ± 1.5 μM) and trehalose (Kd 9.5 ± 1.0 μM). We examined expression of these operons at both the transcriptional and translational levels and found that MalE1 is expressed in cells grown on lactose or guar gum while MalE2 is highly expressed in starch- and trehalose-grown cells. Six other putative sugar binding proteins from examined and four of them were found to be binding to ribose, xylose, myo-inositol and digalacturonic acid with high affinities and specificities. ^