ABSTRACT
The basal genetic potential of diverse cell and tissue types in eukaryotes is similar if not identical. However, to carry out diverse functions, various cells and tissues express only certain genes of the genome. Such precise spatial and temporal regulation of gene expression is important for maintaining highly complex processes of growth and development in a coordinated and orderly manner. Transcription is the first major committed step in gene expression and thus provides a primary and predominant control point for regulation in biological systems. Eukaryotic transcription is regulated in the majority of cases by complex interactions between cis-acting DNA motifs present in the 5' end of the protein-coding regions and trans-acting protein factors. In some cases, the regulatory elements found in the 5' untranslated region and 3' end of the coding region, and even in some of the introns are required for accurate and coordinated gene expression (Larkin et al., 1993; Seiburth and Meyerowitz, 1997; Yu et al., 2001). The DNA sequences present upstream to the transcription start site are collectively referred to as the promoter region and are involved in transcriptional initiation and regulation. Sequences downstream of the coding region are essential for transcription termination and polyadenylation, and in some cases are also necessary for transcription initiation as well (Waugh and Brown, 1991).
