  In this study, through various tests one theoretical model is presented to
describe the relationship between promoter strength and its nucleotide
sequence. Our analysis shows that, promoter strength is greatly influenced by
nucleotide groups with three adjacent nucleotides in its sequence. Meanwhile,
nucleotides in different regions of promoter sequence have different effects on
promoter strength. Based on experimental data for {\it E. coli} promoters, our
calculations indicate, nucleotides in -10 region, -35 region, and the
discriminator region of promoter sequence are more important than those in
spacing region for determining promoter strength. With model parameter values
obtained by fitting to experimental data, four promoter libraries are
theoretically built for the corresponding experimental environments under which
data for promoter strength in gene expression has been measured previously.
