Codon bias- The different codons that correspond to the same amino acid appear at different frequencies in the genome

TLS Online TPP Program

#Id: 556

#Unit 2. Cellular Organization

lacZ = b-galactosidase

lacY = galactoside permease

lacA = galactoside transacetylase

All 3 genes are transcribed together producing 1 mRNA, a polycistronic message 

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#Id: 557

#Unit 2. Cellular Organization

The lacA gene encodes thiogalactoside transacetylase, which rids the cell of toxic thiogalactosides that also get transported by lacY.

TLS Online TPP Program

#Id: 558

#Unit 2. Cellular Organization

There are 4 genes (I, Z, Y, A), 2 Promoters and 3 lac operators in Lac operon

The major O1 lac operator lies adjacent to promoter

Two auxiliary O2 and O3 lac operators 

All three operators are required for optimum repression 

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#Id: 559

#Unit 2. Cellular Organization

The O1 operator extends from position -5 just upstream of the mRNA start point to position +21 within the transcription unit.

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#Id: 560

#Unit 2. Cellular Organization

When polymerase binding causes transition to the open complex and initiates transcription. This gives a low level of constitutive expression called the basal level. 

Binding of RNA polymerase is the rate-limiting step in this case.

TLS Online TPP Program

#Id: 561

#Unit 2. Cellular Organization

lacZ encodes for b-galactosidase, a tetramer of 500 kD. The enzyme breaks lactose is cleaved into glucose and galactose. This enzyme also produces an important byproduct, b-1,6-allolactase.