Overlapping of the coding regions for α and γ components of penicillin-binding protein 1 b in Escherichia coli

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Abstract
The mode of biosynthesis of penicillin-binding protein(PBP)-1 b in Escherichia coli was investigated by use of the plasmid carrying the ponB(PBP-1 b) gene region. Analyses of the products synthesized in minicells and in vitro showed that PBP-1 b was synthesized as two molecular species corresponding to the α and γ components of PBP-1 b. The coding regions for the α and γ components were located within the ca. 3.7 kb MluI-HincII fragment and transcribed in the direction from the HincII to the MluI site. The capacity for producing the α component was abolished by a deletion extending to the MluI site ca. 0.7 kb inward from the HincII end of the ca. 3.7 kb fragment; the remaining 3.0 kb region with the MluI site at both ends directed the production of the γ component alone. The production of the γ component was enough to correct all the known defects caused by a ponB mutation. In addition to these results, the analyses for cross-reacting materials produced in correspondence to the various deletions indicated that the coding regions for the α and γ components overlapped and that the N-terminal portion was responsible for the difference between the two components. The distal region about 0.7 kb long inward from the MluI end of the MluI-HincII fragment was dispensable for producing the functional PBP-1 b, although the PBP-1 b produced was curtailed. By a larger distal deletion reaching almost to the middle of the MluI-HincII fragment, the polypeptide produced for PBP-1 b lost the ability to bind penicillin and still retained a low but significant activity for glycan synthesis. We suggest, therefore, that the polypeptide portion required for transglycosylase activity resides on the N-terminal half of PBP-1 b, followed by the middle portion necessary for penicillin-binding and the C-terminal part dispensable for the function of PBP-1 b.