One bacterial effector with two distinct catalytic activities by different strains.

EMBO reports VOL 14 | NO 9 | 2013 753 occur in mammalian cells and results in global gene transcriptional repression, particularly in genes that are involved in innate immunity. Furthermore, deletion of RomA renders the Paris strain defective in intracellular growth within both macrophages and amoebae, suggesting that repression of host global transcription is important for L. pneumophila pathogenesis. By contrast, Li et al used the L. pneumophila Philadelphia-derivative Lp02 strain to characterize LegAS4. However, unlike RomA from the Paris strain, LegAS4 localizes predominately to the nucleolus in which it catalyses H3K4 methylation to promote increased transcription of rDNA genes through direct interaction with heterochromatin binding protein 1, and does not have a global effect on host gene transcription. Interestingly, Ronaldo et al did not find any evidence for the RomA-mediated H3K4 methylation seen with LegAS4, but did observe an increase in H3K14 methylation in cells infected with the Lp02 strain. This suggests that LegAS4 might catalyse this new histone methylation mark, but that further analyses are needed. How can a highly homologous effector have such discrepant phenotypes in two different strains of L. pneumophila? An amino acid sequence alignment of RomA and LegAS4 reveals two major differences (Fig 1). First, RomA is missing the first 13 amino-terminal amino acids present in LegAS4. Second, an eight amino acid stretch spanning positions 66–74 and 80–87 in RomA and LegAS4, respectively, shows weak sequence homology with only a lysine residue shared in this region (Fig 1). It is possible these two regions of difference, which occur upstream from the conserved catalytic SET domain, might alter the structure of RomA from LegAS4 resulting in a change in histone methyltransferase substrate specificity, thus promoting H3K14 methylation compared with H3K4 methylation. Another factor to consider is the difference in nuclear localization of RomA and LegAS4. Although both RomA and LegAS4 localize to the nucleus, LegAS4 has an intense localization to the nucleolus, which is not seen for RomA. The ability of LegAS4 to localize to the nucleolus is dependent on its tandem nuclear localization signal (NLS) at amino acid positions 21–60, which alone is sufficient to target GFP to the nucleolus. RomA has three individual amino acid differences from LegAS4 in its corresponding NLS (Fig 1). Nuclear and nucleolar localization signals are similar and characterized by stretches of basic amino acids such as lysine [4]. It is possible that the three amino acid differences in the NLS of LegAS4 are sufficient to target this effector to the nucleolus, in which it can act directly on rDNA gene transcription, rather than have an effect on global gene transcription as with RomA, which does not target the nucleolus. Taken together, correspondence correspondence