L-Arabinose transport

V. L-Arabinose Transport, Gene araE, and the Arabinose-Binding Protein 268... 

Direct Selection of Ara Mutants in the araC gene and esi Other Genes Affecting l-Arabinose Transport from araA Mutants... 

Inducibility allele (C" ), the wild-type allele, in which the isomerase, kinase, and epimerase and an L-arabinose transport system are inducible by L-arabinose. 

Pleiotropic constitutive allele (CO, in which the above enzymes and an L-arabinose transport system are produced constitutively—that is, in the absence of inducer. 

The galactose, arabinose and xylose transporters of E. coli The bacterium E. coli possesses at least 7 proton-linked, active transport systems for sugars (for a recent review see [212]). Three of these transporters, which catalyze the uptake of L-arabinose, D-xylose and D-galactose by symport with protons, are related in sequence to the sugar transporters discussed above. They probably represent the best-characterized of the non-mammalian transporters, and so are discussed here in some detail. 

A rather widespread family of proteins, found in the periplasmic space of gramnegative bacteria, complexes certain small molecules and allows them to be transported through the cell wall or activate chemotaxis. Each of these functions involves a consecutive interaction with specific membrane proteins. The molecules transported are amino acids, sulfate, mono- and oligosaccchrides. In this way ABP complexes L-arabinose (K 0.98 x 10 M), and MBP (maltodextrin-binding protein) complexes maltose (ATj 35 x 10 M) and maltodextrins. It is in this series that are found the strongest possible bonds between sugars and proteins. The dissociation rate ( i 1.5 s ) is indicative of the upper limit of the ionic transport rate. Hydrogen bonds... 

The effect of insulin on the kinetics of glucose transport have been studied by Morgan et al. (1961a). They find that insulin increases the Km and Fm x of transport in the perfused isolated rat heart. Fisher and Zachariah (1961) have concluded that insulin increases the Km of L-arabinose and D-xylose transport. In peripheral tissues or in the heart, in vivo evidence has been obtained that insulin lowers the tissue threshold for glucose (i.e., permits uptake of glucose at lower concentration) (Butterfield et al., 1958 Hackcl, 1960). 

L-Arabinose, in Escherichia coli B/r, is converted by a series of consecutive reactions, catalyzed by at least one L-arabinose-induced transport system or permease and three enzymes into D-xylulose 5-phosphate... 

Fig. 1. The L-arabinose gene-enzyme complex showing the L-arabinose operon ara-OIBAD, the L-arabinose regulatory gene araC, and gene araE which is concerned with the active transport of L-arabinose, thr = L-threonine leu = L-leucine thy = thymine ara = L-arabinose.

This is not to imply that the active transport of L-arabinose is not mediated by enzymes. Rather, a distinction is made between those enzymes involved in the chemical conversion of the substrate and those that catalyze reactions involved in active transport in which the substrate is not obviously (or irreversibly) altered as the result of transport. 

There is evidence for the existence of yet another L-arabinose-inducible permease system which transports L-arabinose and is probably controlled by an additional regulatory gene other than gene araC. Because of the limitation of space and the fact that our knowledge of the permease operons is very limited, this article will be primarily concerned with the regulation of the OIBA D operon. Unless otherwise indicated, when reference is made to the L-arabinose operon, I will be referring to the OIBAD operon. 

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