  A major challenge in synthetic genetic circuit development is the
inter-dependency between heterologous gene expressions by circuits and host's
growth rate. Increasing heterologous gene expression increases burden to the
host, resulting in host growth reduction; which reduces overall heterologous
protein abundance. Hence, it is difficult to design predictable genetic
circuits. Here, we develop two biophysical models; one for promoter, another
for RBS; to correlate heterologous gene expression and growth reduction. We
model cellular resource allocation in E. coli to describe the burden, as growth
reduction, caused by genetic circuits. To facilitate their uses in genetic
circuit design, inputs to the model are common characteristics of biological
parts [e.g. relative promoter strength (RPU) and relative ribosome binding
sites strength (RRU)]. The models suggest that E. coli's growth rate reduces
linearly with increasing RPU / RRU of the genetic circuits; thus, providing 2
handy models taking parts characteristics as input to estimate growth rate
reduction for fine tuning genetic circuit design in silico prior to
construction. Our promoter model correlates well with experiments using various
genetic circuits, both single and double expression cassettes, up to a relative
promoter unit of 3.7 with a 60% growth rate reduction (average R2 ~ 0.9).
