Functional analysis of PilT from the toxic cyanobacterium Microcystis aeruginosa PCC 7806

School of Biotechnology and Biomolecular Sciences, The University of New South Wales, Sydney, NSW 2052, Australia

^* To whom correspondence should be addressed. Email: b.neilan{at}unsw.edu.au.

	Abstract

The evolution of the microcystin toxin gene cluster in phylogenetically distant cyanobacteria has been attributed to recombination, inactivation, and deletion events, although gene transfer may also be involved. Since the microcystin-producing Microcystis aeruginosa PCC 7806 is naturally transformable, we have initiated the characterization of its type IV pili system, involved in DNA uptake in many bacteria, to provide a physiological focus for the influence of gene transfer in microcystin evolution. The type IV pili genes pilA, pilB, pilC and pilT were shown to be expressed in M. aeruginosa PCC 7806. The purified PilT protein yielded an maximal ATPase activity of 37.5 ± 1.8 nmol P_i min^-1 mg^-1 protein, with a requirement of Mg²⁺. Heterologous expression indicated that it could complement the pilT mutant of P. aeruginosa, but not that of the cyanobacterium Synechocystis sp. strain PCC 6803, which was unexpected. Differences in two critical residues between the M. aeruginosa PCC 7806 PilT (7806PilT) and the Synechocystis sp. strain PCC 6803 PilT proteins affected their theoretical structural models, which may explain the non-functionality of 7806PilT in its cyanobacterial counterpart. Screening of the pilT gene in toxic and non-toxic strains of Microcystis was also performed.