The limb bud is regionalized into 3 functional domains

1. Progress zone

2. Zone of polarizing activity

3. Apical ectodermal ridge

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 

TLS Online TPP Program

#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 

TLS Online TPP Program

#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.

TLS Online TPP Program

#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.