The Ivh Type IVA Secretion System

The second Legionella type IV secretion system, Ivh (Legionella VirB homologues), is dispensable for intracellular growth in both macrophages and amoebae (Segal et al. 1999), but is implicated in host cell infection by L. pneumophila at 30 °C (Ridenour et al. 2003), and it can also mediate conjugational DNA transfer (Segal et al. 1999). This secretion system is constituted of 11 genes, which are located on a plasmid-like element that can exist either integrated in the chromosome or excised as a multicopy plasmid in strains Paris and Philadelphia 1, but seems to be stably integrated in strain Lens. However, a 28-bp perfect repeat is present in the flanking regions in strain Lens, suggesting that it is also able to excise. This element is probably of plasmid origin as it has a higher G+C content than the rest of the chromosome and it carries some plasmid-related genes. The elucidation of the biological implications of these different forms is another challenging question to answer.

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