Molecular Biology and Evolution, Vol 5, 469-485, Copyright © 1988 by Society for Molecular Biology and Evolution
AM Dean, DE Dykhuizen and DL Hartl
Two genetic procedures were used to obtain amino acid replacements in the
lacZ-encoded beta-galactosidase in Escherichia coli. Amino acid
replacements could be obtained without regard to their effects on lactase
activity by selecting spontaneous mutations that relieved the strong
polarity of six nonsense mutations. When streaked on MacConkey- lactose
indicator plates, approximately 75% of these mutants gave strong red
lactose-fermenting colonies, and 25% gave white nonfermenting colonies.
Mutants from 11 other nonsense codons were isolated directly using
MacConkey-lactose indicator plates, on which positive color indication
requires only 0.5% of the wildtype lactase activity. Among the total of 17
codons, 25 variant beta-galactosidases were identified using
electrophoresis and thermal denaturation studies. The fitness effects of
these variant beta-galactosidases were determined using competition
experiments conducted with lactose as the sole nutrient limiting the growth
rate in chemostat cultures. Three of the replacements were deleterious, one
was selectively advantageous, and the selective effects of the remaining 21
were undetectable under conditions in which the smallest detectable
selection coefficient was approximately 0.4%/generation.
ORIGINAL ARTICLE
Fitness effects of amino acid replacements in the beta-galactosidase of Escherichia coli
Department of Genetics, Washington University School of Medicine, St. Louis, Missouri 63110-1095.
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