The production of hemagglutinins, which are bacterial adhesins for mammalian cells, is a wellestablished virulence factor for a number of bacterial pathogens. Thus, it was not surprising that...

The production of hemagglutinins, which are bacterial adhesins for mammalian cells, is a wellestablished virulence factor for a number of bacterial pathogens. Thus, it was not surprising that Porphyromonas gingivalis, a gram-negative anaerobic bacterium associated with dental plaque and periodontal disease, also expresses several hemagglutinins on its cell surface. It was found that one of the hemagglutinin genes (hagA) of P. gingivalis contains four large, contiguous direct repeats varying from 1,318 to 1,368 bp in length, which together encode a 2,628-amino-acid protein of 283.3 kDa (a very large protein!). The repeat unit (denoted HArep), which contains the hemagglutinin adhesin domain, was also found to be present in several other protease and hemagglutinin genes in P. gingivalis. The beginning amino acid sequence encoded by the first repeat (HArep1) is PNPNPGTTT, while that of the other three repeats (HArep2 to -4) is GTPNPNPNPNPGTTT. The amino acid sequence at the C terminus of the fourth repeat (HArep4) is GTPNPNPNP. Provide a possible mechanism that could account for the presence of this repeat unit four times in hagA and also in the other protease and hemagglutinin genes (from a molecular evolutionary point of view).