The bacterial lactose converting enzyme beta-galactosidase (β-gal) and its gene (LacZ) have been studied for many years (1 and references therein) and are among the most utilized tools in molecular biology. The LacZ product, a polypeptide of 1029 amino acids, gives rise to the functional enzyme after tetrameriz-ation (2 and references therein) and is easily detected by chromogenic substrates either in cell lysates or directly on fixed cells in situ (3 and references therein). The tetramerization isdependent on the presence of the N-terminal region spanning the first 50 residues (2 and references therein). Deletions in the N-terminal sequence generate a so-called omega peptide that is unable to tetramerize and does not display enzymatic activity. The activity of the omega peptide can be fully restored either in bacteria or in virro (4) if a small fragment (called alpha peptide) corresponding to the intact N-terminal portion of the β-gal is added in trans. The phenomenon is called alpha complementation and the small N-terminal peptide is called alpha peptide. This effect has been widely exploited for studies in procaryotes, where special strains that constitutively express omega peptide exist and allow the detection of expression of the small alpha peptide.
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