Intrahelical pseudoknots and interhelical associations mediated by mispaired human minisatellite DNA sequences in vitro.

The human minisatellite arrays, 33.6 and 33.15, consist of tandem reiterations of a 37-nucleotide (nt) and a 16-nt repeat unit sequence, respectively, both of which contain a majority of purine bases on one strand. Knot-like tertiary structures, which mapped to the cloned arrays, were observed by electron microscopy (EM) in homoduplex molecules produced by denaturation and reannealing in vitro. They result from a primary hybridization between misaligned repeat units of the array, forming a slipped-strand structure with staggered single-stranded DNA loops, followed by a secondary hybridization between repeat units in the two loops. Depending on the relative alignment of the loops when they hybridize, a particular form of intrahelical pseudoknot is produced. Theta-shaped, figure-of-eight, and bow-shaped structures were the most common conformational isomers observed in homoduplexes flattened into two dimensions during EM preparation. At the site of a bow-shaped structure, a conformation-dependent bend of approximately 60 degrees between the flanking DNA segments is induced; the other conformations generally do not deflect the line of the main DNA axis. Paired loops, similar to the bow-shaped structure, were apically situated in some supercoiled plasmids containing the 33.6 array. Both plasmids formed intermolecular associations, consisting of two (or more) homoduplex molecules held together at or immediately adjacent to a nexus which mapped to the minisatellite sequences. These associations might arise either by interhelical hybridization between arrays or by knot-like structures interfering with branch migration of chi-form Holliday junctions.     

Plant Growth Regulators in Citriculture: World Current Uses

Plant growth regulators (PGRs), both naturally occurring and synthetic, are used extensively in citriculture not only to help growers solve production problems, but also to increase the market value of the crop and to improve grower returns. Yield increases are obtained via an improvement in fruit set, particularly for shybearing cultivars, and/or fruit size, especially for the “small-fruit” group of mandarins. Increased fruit number per tree is achieved indirectly through an increase in flower number as a result of better flower initiation or by a direct effect on fruit set. Improved fruit size is brought about directly by stimulating the growth of fruit tissues or indirectly by reducing fruit number by partial inhibition of flower initiation or by subsequent fruit removal. PGRs are also used to hasten maturity, to delay harvest, and to maintain fruit quality postharvest, resulting in greater profits to the grower. This article provides an overview of current uses of PGRs in citriculture. The reported information is based on an international survey and a review of recent technical and scientific literature published on the subject. Procedures and techniques used to enhance the efficacy of PGRs, including surfactants and compatibility considerations, are presented and discussed. The detailed information presented in the text is summarized in tables designed to provide a convenient reference for researchers and growers alike. Included in the tables are the names of the individuals who were the source of the information. A comprehensive list of these individuals and their addresses follows the tables. Whereas the results of the survey indicate extensive uses of PGRs in citriculture, the authors admit that the survey is incomplete and extend their apologies to researchers in countries not represented in the review and for inadvertent omissions.