Beta-galactosidase

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Revision as of 16:49, 18 October 2007

Some interesting facts [1]

• A tetramer of 4 identical subunits
• Each subunit is 120kD.
• Active only as a tetramer.
• Mutations in some of the codons of the N-terminal 60aa or C-terminal 100aa results in an inactive, dimeric β-galactosidase. [2]
• If the above sequences are deleted, the missing protein fragment can be replaced by the corresponding peptide. This is called intracistronic alpha or omega complementation respectively.
• Its N-terminal 23 residues can be replaced by any amino acid residues without affecting the enzymatic activity.
• A mutant with an internal deletion of codons 21-41 of the lacZ gene does not produce any active β-galactosidase.
• A mutant with a deletion of everything past residue 60 (i.e. it expresses only the first 60 N-terminal amino acids) does not produce any active β-galactosidase.
• You can measure lacZ activity using flow cytometry. See A flow cytometric study of stationary phase gene expression in E.coli using lacZ reporter gene fusion
  • This paper uses C₁₂FDG from Invitrogen. However, FDG might be a better substrate in gram-negative bacteria like Escherichia coli [3]
• Alpha-complementation of β-galactosidase does not seem to yield activities equal to wildtype β-galactosidase. Depending on the fragment, the activity can be up to 24% of wildtype [4]. (If anyone has a better reference comparing results from a Miller assay of alpha-complementated β-galactosidase with wildtype, please include it here.)

Protocols

1. Beta-Galactosidase Assay (A better Miller)
2. BE.109:Protein engineering/Assessing beta-galactosidase

Reference

1. Benno Muller-Hill. The Lac Operon. Walter de Gruyter. isbn:3-11-014830-7. [Muller-Hill-1996]
2. isbn:0317118099. [Beckwith-1970]
3. Plovins A, Alvarez AM, Ibañez M, Molina M, and Nombela C. . pmid:7811104. PubMed HubMed [Plovins-ApplEnvironMicrobio-1994]
4. Zamenhof PJ and Villarejo M. . pmid:4552986. PubMed HubMed [Zamenhof-JBacteriol-1972]
5. Ullmann A. . pmid:1345751. PubMed HubMed [Ullmann-Bioessays-1992]
6. [by] Jeffrey H. Miller. Experiments in molecular genetics. [Cold Spring Harbor, N.Y.] Cold Spring Harbor Laboratory, 1972. isbn:0879691069. [Miller-1972]

   original β-galactosidase assay by Miller