Amino acid sequence homology between N- and C-terminal halves of a carbonic anhydrase in Porphyridium purpureum, as deduced from the cloned cDNA

Abstract

Carbonic anhydrase (CA) from Porphyridium purpureum, a unicellular red alga, was purified >209-fold to a specific activity of 1,147 units/mg protein. cDNA clones for this CA were isolated. The longest clone, comprising 1,960 base pairs, contained an open reading frame which encoded a 571-amine acid polypeptide with a calculated molecular mass of 62,094 Da. The N- and C- terminal halves of the putative mature Porphyridium CA have amine acid sequence homology to each other (>70%) and to other prokaryotic-type CAs. Both regions contain, at equivalent positions, one set of three possible zinc-liganding amine acid residues conserved among prokaryotic-type CAs. CA purified from Porphyridium contained two atoms of zinc per molecule. We propose that the Porphyridium CA has evolved by duplication of an ancestral CA gene followed by the fusion of the duplicated CA gene. The CA truncated into the putative mature form was overexpressed in Escherichia coli, and the expressed protein was active. Clones expressing separately the N- and C- terminal halves of the CA were constructed. CA activity was present in extracts of E. coli cells expressing the N-terminal half, while no detectable activity was found in cells expressing the C-terminal half.