Contiguous repeats

The changes in transcription are due to the presence of heat shock elements (HSE) in heat shock gene promoters. In E. coli, the consensus sequence for the HSE is CTGCCACCC at nucleotide positions -44 to -36 relative to the transcription initiation site. In eukaryotes, the HSE consist of contiguous repeats (in alternating orientation) of the S-base pair sequence NGAANN (N = any nucleotide), upstream of the TATA box. 

Let if be the number of pearls that consist of a number C of contiguous repeat units, and let be the number of swollen hydrated coils of the length f connecting them. Chain conformation is specified by the indices i = /i,/2, - and j = as shown in... 

The -B-A-A- and -A-A-B- triads are indistinguishable because chain direction cannot be specified. The same applies for -A-B-B- and -B-B-A-. (For diads, each repeat unit is part of two contiguous repeat unit sequences, tetrads are viewed according to their diad centers and pentads are identified by their triad centers.)... 

Trinucleotide Repeat Expansion DNA region comprised of a variable number of repetitive, contiguous trinucleotide sequences. The presence of these regions is associated with diseases such as Fragile X Syndrome and myotonic dystrophy. Many chromosome fragile sites (chromosome fragility) contain expanded trinucleotide repeats, [nih]... 

Fibrous protein sequences are often characterized by the presence of simple repetitive motifs. Some are exact in length and/or sequence, but others are only approximate and display considerable variation. Some motifs contain residues that are absolutely conserved in some positions, whereas in others it is only the sequence character that is maintained over the repeat length. In many fibrous proteins the repeats occur contiguously, whereas in others they are found widely separated in the sequence. The varieties of sequence repeat that have been observed are typed and catalogued here by Parry (Chapter 2). Each motif forms a discrete element of structure in many instances, these are arranged helically with respect to one another. In many cases an elongate structure is formed, and this can lead naturally to molecular aggregation and the formation of functional filaments. 

The classic example of a Type B repeat is that presented by the o-chains in collagen. Three such chains aggregate to form a triple-helical collagen molecule, but they can only do so if glycine is positioned in every third residue of the sequence (Hulmes, 1992). This is because glycine is located internally and, due to its size, is the only residue that can fit stereochemi-cally into the space available. In the Type I collagen Q-chain, the repeat occurs 338 times contiguously. 

The tail domain in trichocyte Type I chains contains a tenfold P-C-X repeat, seven of them contiguously (Parry and North, 1998). Studies of its likely conformation suggest that it will most likely adopt a left-handed polyglycine II structure with three residues per turn (Fig. 3a). The cysteine residues would thus lie along one edge of the structure, where they would be in positions to form disulphide bonds with cysteine residues in a similar structure from a different molecule. The role of this sequence motif would appear to be that of stabilizing molecular assembly within the IFs. 

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