Research Article 4: Structural Studies of ω-complemented β-galactosidase of Escherichia coli

Abstract

In view of the fact that the kinetic parameters of the β-galactosidase reaction exhibited by wild-type and ω-complemented β-galactosidases (Ullmann et al., 1965) are not significantly different, while their structure could hardly be the same, comparative physical studies of these two types of enzymes have been undertaken (Goldberg, 1969; Goldberg et al., 1969). These studies were performed with pure ω-complemented enzyme extracted from the diploid strain MU366/B9 carrying on the chromosome a nonsense mutation within the ω segment and on the episome a z deletion extending to very near the operator proximal boundary of the genetic ω segment The authors came to the conclusion that, like the wild-type enzyme (Craven et al., 1965) this particular ω-complemented enzyme is a tetramer; its protomer was shown to contain two different polypeptide chains: one of mol wt 110,000 likely to be the acceptor and the other of mol wt 39 000, presumably the ω. These findings led to the conclusions that the region of the gene which lies between the ω barrier (I) and the MU366 mutation is effectively translated into a sequence of amino acids both in the C-terminal part of the acceptor and in the N-terminal part of the ω.

This C-terminal part of the acceptor polypeptide chain can be released from the complemented enzyme by a mild proteolytic treatment without loss of β-galactosidase activity. This suggests that the C-terminal part of the acceptor: (a) is not essential for the enzymatic activity of the complete complemented molecule (b) is presumably located...