Protein sequence pool

The Rev protein of HIV-1 facilitates the nuclear export of incompletely spliced viral mRNAs and plays, therefore, an important role in the production of viral structural proteins. Rev specifically binds to a responsive element which folds into a stem-internal loop-stem secondary structure, the Rev-binding element (RBE) located into the rev gene. In vitro selection has been used to determine interactions between Rev and the RBE. RNA motifs which could bind Rev up to ten-fold better than the wild-type sequence have been isolated either from an RNA library constituted of partly randomized RBE (Bartel et al., 1991) or from completely random sequence pools, based on the RBE secondary structure (Giver et al., 1993 Tuerk and MacDougal-Waugh, 1993). Novel RNA sequences and secondary structural motifs have been selected. In particular, a wild-type G G pair is frequently replaced by an A A or even by a C A pair which are isosteric (Giver etal., 1993). 

Nucleic acids that can perform a wide variety of binding reactions have been selected from random sequence pools by affinity immobilization. Oliphant et al. [2] selected DNA molecules that could bind to the yeast transcriptional activator GCN4 from a random-sequence DNA pool that spanned nine positions. Since then, aptamers (nucleic acid ligands) have been selected against a variety of protein targets that naturally bind to nucleic acids, such as EF-Tu, ribosomal proteins, QP replicase, and reverse transcriptase (reviewed in Ref. 3). In addition, aptamers have been selected against intracellular and... 

Finally, novel nucleic acid catalysts have also been selected from random sequence pools (reviewed in Ref. 19). Joyce and co-workers have manipulated the function of the Group I self-splicing ribozyme, selecting variants that can utilize calcium or cleave DNA from partially randomized pools [20,21], Lorsch and Szostak [22] selected a polynucleotide kinase ribozyme from a completely random sequence pool that flanked a previously selected ATP binding site. Many of the novel ribozymes can catalyze reactions that are relevant to protein biosynthesis, bolstering arguments that translation may have arisen in a putative RNA world. For example, Lohse and Szostak [23] have selected ribozymes that can carry out an acyl transfer reaction, while Illangasekare et al. [24] have isolated a... 

Aptamer Aptamers are oligonucleic acid or peptide molecules that bind to a specific target molecule. Aptamers are usually created by selecting them from a large random sequence pool, but natural aptamers also exist in riboswitches. Aptamers can be combined with ribozymes to self-cleave in the presence of their target molecule. DNA or RNA aptamers consist of (usually short) strands of oligonucleotides, while peptide aptamers consist of a short variable peptide domain, attached at both ends to a protein scaffold. 

Both PSI and PSII are necessary for photosynthesis, but the systems do not operate in the implied temporal sequence. There is also considerable pooling of electrons in intermediates between the two photosystems, and the indicated photoacts seldom occur in unison. The terms PSI and PSII have come to represent two distinct, but interacting reaction centers in photosynthetic membranes (36,37) the two centers are considered in combination with the proteins and electron-transfer processes specific to the separate centers. 

---