1. Chromatography
Thin-layer chromatography (TLC), in which the adsorbent is in a thin and uniform layer fixed on a suitable supporting plate of a material, such as glass or plastic, was first suggested by Izmailov and Shraiber in 1938 (1). This type of chromatography was not widely used until the late 1950s, probably because the development of paper and gas-liquid chromatography was proceeding rapidly at that time. Its rapid development began about 1956, mainly because of the work of Stahl (2, 3), who devised convenient methods for preparing plates and showed that TLC is applicable to a wide variety of separations.
In thin-layer chromatography, a solution of the sample in a volatile solvent is applied to the bottom of a TLC plate. When the spot has dried, the plate is placed vertically in a suitable tank with its lower edge immersed in the selected mobile phase. The mobile phase rises by capillary action, producing an ascending chromatographic separation and resolving the various components of the sample mixture into discrete spots. At the end of the run, the mobile phase is allowed to evaporate from the plate, and the separated spots are located and identified by physical and/or chemical methods.
Thin-layer chromatography has the advantage of performing a separation easily in a minimum of time and with a minimum of chemicals and instrumentation. TLC also offers good selectivity and a wide variety of possible chromatographic interactions. TLC run in a two-dimensional mode has more separating power than the well-developed HPLC method. Recent developments in TLC instrumentation make the field more and more exciting and provide increased possibilities for performing this technique with high accuracy (4).
2. Electrophoresis
The basis of electrophoresis is the differential migration rate of ionic molecules in an electrolyte solution under the influence of an applied electric field. Although electrophoresis is not in principle a chromatographic technique, it is used in conjunction with paper chromatography, and it provides an extremely useful method for separating charged substances, such as proteins and nucleic acids. Many forms of electrophoresis are carried out in gel media. Electrophoresis carried out on cellulose or paper strips is called zone electrophoresis. The capillary walls provide mechanical support for the carrier electrolyte in capillary zone electrophoresis. Detailed practice in electrophoresis is beyond the scope of this article, so interested readers are directed to a number of excellent monographs (5, 6).
Both early and middle promoters require host s . Binding of AsiA protein to s prevents transcription of early T4 promoters with "standard" -35 regions, but infection can proceed because most prereplicative genes are also transcribed from middle promoters (11) (Fig. 3b). At middlepromoters, the RNA polymerase, whose s -subunit has acquired AsiA protein, recognizes consensus -10 regions and T4 MotA protein bound to consensus DNA sequences called motA boxes (11, 32, 33) (Fig. 3). Transcriptions from early and middle promoters can overlap in time, another
