Directly repeated orientation

A distinctive feature of the recombination reaction mediated by Tn3 and yS resolvases is the requirement for two res sites present as direct repeats (Reed and Grindley, 1981 Kitts etal., 1983). DNA relaxation by Tn3 resolvase also occurs efficiently only on DNA molecules containing two res sites in a directly repeated orientation (Krasnow and Cozzarelli,... 

It is believed that many transcription factors bind DNA as dimers ( dimeric transcription factors) in either the same or opposite orientation. The DNA sites therefore can look like two direct repeat sequences or like palindromes. The interaction of the subunits with each other must obviously be specific and be mediated by dimerization domains. Specificity and stability of the dimers (dimeric transcription factors) is mostly promoted by hydrophobic or ionic inteiphases, e.g. a... 

These appear in both 5 to 3 and 3 to 5 orientations, allowing them to form two base paired "stems." In addition, there are three direct repeats of a 13-residue consensus sequence 5 -GATCTNTTNTTTT (shaded in green in Fig. 27-18), which form an AT rich duplex. Other bacterial replication origins often follow a... 

RAR-RXR heterodimeric complexes bind to a directly repeated, hexanucleotide motif of the consensus sequence 5 -PuGGTCA-3 (where Pu represents a purine, see above). Directly repeated response elements, in contrast to inverted repeats, are not symmetrical, and this renders the microenvironment of each half-site distinct (38). This situation implies that the orientation of RAR-RXR complexes RAREs may be ordered and occur in a non-random manner. Indeed, this is the case Perl-mann and colleagues used biochemical tech-... 

The length of the direct repeats is constant for a given IS element, but their sequence depends on the site of insertion and therefore varies with each transposition of the IS element. Arrows indicate sequence orientation. The regions in this diagram are not to scale the coding region makes up most of the length of an IS element. 

If a shallow kick off in soft formation is required (e.g. to steer the borehole away underneath platforms) a technique using jet bit deflection or badgering is employed (Fig. 3.16). A rock bit is fitted with two small and one large jet. With the bit on bottom and oriented in the desired direction the string is kept stationary and mud is pumped through the nozzles. This causes asymmetric erosion of the borehole beneath the larger jet. Once sufficient hole has been jetted, the drill bit will be rotated again and the new course followed. This process will be repeated until the planned deviation is reached. 

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