Isolation and sequencing of a new beta-galactosidase-encoding archaebacterial gene

The gene lacS coding for a beta-galactosidase (beta Gal; EC 3.2.1.23) has been cloned from the thermoacidophilic archaebacterium Sulfolobus solfataricus, strain MT-4. It encodes a polypeptide chain of 489 amino acids (aa) (56,764 Da) in good agreement with the value directly measured for the enzyme (60 +/- 2 kDa per subunit). The aa composition of the enzyme and, in particular, its peculiarly low cysteine content (one Cys per subunit) has been confirmed; at the same time, it has been observed that the very low G + C content of the S. solfataricus genome strongly influences the codon usage preferences in the lacS sequence. There appears to be no evident similarity between this and the Escherichia coli lacZ sequence, thus suggesting that the two enzymes have analogous function, but are not homologous. By comparison with the published sequences of archaebacterial promoters, terminators and ribosome-binding sites, potential regulatory sites have been identified in the flanking regions of the S. solfataricus lacS gene.