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A new labelling method for cloned DNA probes used in hybridization assays is described. The DNA insert of recombinant plasmid DNA was made partially single-stranded for the labelling reaction by a restriction enzyme digest, followed by a controlled exonuclease III incubation. A thiol-containing psoralen derivative was covalently bound through irradiation with UV-light to the remaining double-stranded region of the plasmid DNA. The psoralen-SH groups were labelled with a large number of metal chelators (diethylentriamine pentaacetic acid, DTPA) using poly-L-lysine as a macromolecular carrier. The main advantage of the labelling procedure is that a high degree of labelling is achieved without modification of the single-stranded DNA hybridizing sequences. The specific hybrids were labelled after filter hybridization with europium ions through the chelating groups of DTPA. The europium ions were quantitatively detected by time-resolved fluorometry. The sensitivity of the assay for target DNA detection was in the low picogram range, comparable to radioactively labelled DNA probes.

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Maltooligosaccharides with two to six (al-4)-linked glucose residues, carrying at their reducing end a 3-azi-lmethoxybuty! group in either 01 or in B glycosidic linkage, were synthesized. These maltooligosaccharide analogues inhibit maltose uptake via the maltose-binding-protein-dependent transport system in Escherichia coli. The concentration of half-maximal inhibition of maltose transport, at 15 nM concentration, decreases with increasing chain length of the analogue, levelling off at 40 pM after a chain length of four glucose residues in the a series Thus, 3-azi-l-methoxybutyl a-D-maltooligosaccharides are potent photoaffinity labels for proteins with maltooligosaccharides-binding sites.Escherichia coli contains an intricate system for the utilization of maltose and maltooligosaccharides. There are five proteins that represent the complex transport machinery [I]. Fig. 1 shows schematically the position of the corresponding proteins in the cell envelope. LamB, the receptor for phage lambda [2], establishes a diffusion pore [3, 41 with a remarkable specificity for maltodextrins [5] across the outer membrane. The water soluble maltose-binding protein (MBP) that establishes the essential recognition site of the overall transport system [6] is located in the periplasm. It also serves as the chemoreceptor for maltose chemotaxis ["I. MBP binds maltose, maltooligosaccharides [8] and even amylose 191 with an affinity in the micromolar range. It undergoes a conformational change upon binding substrate [3]. In analogy to other transport-related periplasmic substrate-binding proteins 110, 111, it is likely that bound substrate becomes inaccessible to the bulk solvent. The primary sequence of MBP

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