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Unformatted text preview: structural gene mutations are usually independent
• Partial diploids were made by conjugation with an F’ factor
containing the lac operon
• Partial diploids with lacZ+ lacY- on the bacterial chromosome
and lacZ- lacY+ on the plasmid (written: lacZ+ lacY- / lacZ- lacY+):
• functioned normally, producing !-galactosidase and
permease in the presence of lactose.
• Are these mutations dominant, or recessive? recessive because if given 2 copies, The lacI gene is trans dominant
no repressor= always on (constitutive) lacI+ lacZ- / lacI- lacZ+ work in trans, proteins are mobile,
dna seq are not mobile
with only the lacI- allele, how In bacteria
would you describe transcription?
lackI is transdominant Superrepressor mutations (lacIS) prevent
transcription even in the presence of lactose
irrever. binds cant let go, transc. is always off, mutation in entire gain of function pheno typ
56:30 Lac mutations and
different components of the operon G ROUP 2: REGULATORY MUTATIONS
2. Regulatory mutations affect the entire operon (expression of all
structural genes is altered)
• Regulatory mutations affect two sequences:
• The promoter (lacP)
• The operator (lacO) Promoter mutations
• Mutations in the promoter site are designated lacP- no promoter • lacP- mutations prevent binding of RNA polymerase to the promoter.
• Bacteria with lacP- mutations do not produce lac enzymes either in
the presence or absence of lactose. cant bind rna polym, loss of function • Would you expect these mutations to be trans- or cis-acting?
cis Promoter mutations
• The partial diploid:
– lacI+ lacP+ lacZ+ / lacI+ lacP- lacZ+ has normal synthesis of !galactosidase
– lacI+ lacP- lacZ+ / lacI+ lacP+ lacZ- fails to produce !-galactosidase
whether or not lactose is present. no promoter attached to functional gene Operator mutations
• Mutations in the operator site are designated lacOc operator constitutive T • lacOc mutations prevent binding of the lacI repressor to the operator.
• Bacteria with lacOc produce lac enzymes constitutively.
lacOc only affects genes on same chromosome • Would you expect these mutations to be trans or cis acting?
dna seq mut are cis acting Operator mutations are
constitutive and cis-acting Genetics of the Lac Operon
Genotype Lactose No Lactose I+O+Z+ (wildtype) + - I+O+Z- - - mutation in Bgal muta in repressor I- O+Z+ + + I+OCZ+ + + I+OCZIS O+Z+ - - always activity
bgal always on
always off Partial Diploids
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