Haloarchaeal myovirus φCh1 harbours a phase variation system for the production of protein variants with distinct cell surface adhesion specificities.

The φCh1 myovirus, which infects the haloalkaliphilic archaeon Natrialba magadii, contains an invertible region that comprises the convergent open reading frames (ORFs) 34 and 36, which code for the putative tail fibre proteins gp34 and gp36 respectively. The inversion leads to an exchange of the C-termini of these proteins, thereby creating different types of tail fibres. Gene expression experiments revealed that only ORF34 is transcribed, indicating that φCh1 produces tail fibre proteins exclusively from this particular ORF. Only one of the two types of tail fibres encoded by ORF34 is able to bind to Nab. magadii in vitro. This is reflected by the observation that during the early phases of the infection cycle, the lysogenic strain L11 carries its invertible region exclusively in the orientation that produces that specific type of tail fibre. Obviously, Nab. magadii can only be infected by viruses carrying this particular type of tail fibre. By mutational analysis, the binding domain of gp34 was localized to the C-terminal part of the protein, particularly to a galactose-binding domain. The involvement of galactose residues in cell adhesion was supported by the observation that the addition of α-D-galactose to purified gp34 or whole virions prevented their attachment to Nab. magadii.