The second cholera toxin, Zot, and its plasmid‐encoded and phage‐encoded homologues constitute a group of putative ATPases with an altered purine NTP‐binding motif

It is shown that the second cholera toxin, Zot, ORF3 product of Pseudomonas plasmid pKB740, and ORF424 product of bacteriophage Pfl are a group of closely related proteins containing a modified version of the purine NTP‐binding motif, with a drastic substitution of tyrosine for a conserved glycine. They are distantly but reliably related to the product of gene I of filamentous bacteriophages which is a putative ATPase containing the classical NTP‐binding motif and is involved in bacteriophage assembly and exit from the bacterial cell. Hydropathy analysis suggests that the Zot and gene I product may have a similar transmembrane topology. It is hypothesized that Zot may possess ATPase activity and modify the membrane structure of its target cells in an ATP‐dependent fashion. Genes for Zot and the related protein of pKB740 are likely to have evolved from gene I of a Pf1‐like bacteriophage.