We have prepared a library of -106 different peptide sequences on small, spherical (10-pm diameter) beads by the combinatorial chemical coupling of both L-and D-amino acid building blocks. To each bead is covalently attached many copies of a single peptide sequence and, additionally, copies of a unique single-stranded oligonucleotide that codes for that peptide sequence. The oligonucleotide tags are synthesized through a parallel combinatorial procedure that effectively records the process by which the encoded peptide sequence is assembled. The collection of beads was screened for binding to a fluorescently labeled anti-peptide antibody using a fluorescence-activated cell sorting instrument. Those beads to which the antibody bound tightly were isolated by fluorescenceactivated sorting, and the oligonucleotide identifiers attached to individual sorted beads were amplified by the PCR. Sequences of the amplified DNAs were determined to reveal the identity of peptide sequences that bound to the antibody with hig afminty. By combining the capacity for information storage in an oligonucleotide code with the tremendous level of amplification possible through the PCR, we have devised a means for specifying the identity of each member of a vast library of molecules synthesized from both natural and unnatural chemical building blocks. In addition, we have shown that the use of flow cytometry instrumentation permits facile isolation of individual beads that bear high-afflmity ligands for biological receptors.

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The arginine at position 200 of EcoRI endonuclease is thought to make two hydrogen bonds to the guanine of the sequence GAATTC and thus be an important determinant of sequence discrimination. To elucidate the molecular basis of one example of the interaction of proteins with specific sequences of DNA, we are investigating the DNA binding and cleavage mechanisms of the EcoRI endonuclease. The catalytically active form of the endonuclease is a highly stable dimer composed of identical 31-kDa subunits of known amino acid sequence. The enzyme recognizes the double-stranded DNA sequence 5'-GAATTC-3' and cleaves the phosphodiester bonds on both strands between the G and A residues. Mg2" is required for cleavage; in its absence the enzyme forms a stable sequence-specific complex with DNA containing the substrate site (Kd 10 pM) (1, 2). The endonuclease is found in