AUTHOR=Pesce Céline , Jacobs Jonathan M. , Berthelot Edwige , Perret Marion , Vancheva Taca , Bragard Claude , Koebnik Ralf 

TITLE=Comparative Genomics Identifies a Novel Conserved Protein, HpaT, in Proteobacterial Type III Secretion Systems that Do Not Possess the Putative Translocon Protein HrpF

JOURNAL=Frontiers in Microbiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.01177

DOI=10.3389/fmicb.2017.01177

ISSN=1664-302X

ABSTRACT=<p><italic>Xanthomonas translucens</italic> is the causal agent of bacterial leaf streak, the most common bacterial disease of wheat and barley. To cause disease, most xanthomonads depend on a highly conserved type III secretion system, which translocates type III effectors into host plant cells. Mutagenesis of the conserved type III secretion gene <italic>hrcT</italic> confirmed that the <italic>X. translucens</italic> type III secretion system is required to cause disease on the host plant barley and to trigger a non-host hypersensitive response (HR) in pepper leaves. Type III effectors are delivered to the host cell by a surface appendage, the Hrp pilus, and a translocon protein complex that inserts into the plant cell plasma membrane. Homologs of the <italic>Xanthomonas</italic> HrpF protein, including PopF from <italic>Ralstonia solanacearum</italic> and NolX from rhizobia, are thought to act as a translocon protein. Comparative genomics revealed that <italic>X. translucens</italic> strains harbor a noncanonical <italic>hrp</italic> gene cluster, which rather shares features with type III secretion systems from <italic>Ralstonia solanacearum</italic>, <italic>Paraburkholderia andropogonis</italic>, <italic>Collimonas fungivorans</italic>, and <italic>Uliginosibacterium gangwonense</italic> than other <italic>Xanthomonas</italic> spp. Surprisingly, none of these bacteria, except <italic>R. solanacearum</italic>, encode a homolog of the HrpF translocon. Here, we aimed at identifying a candidate translocon from <italic>X. translucens</italic>. Notably, genomes from strains that lacked <italic>hrpF</italic>/<italic>popF</italic>/<italic>nolX</italic> instead encode another gene, called <italic>hpaT</italic>, adjacent to and co-regulated with the type III secretion system gene cluster. An insertional mutant in the <italic>X. translucens hpaT</italic> gene, which is the first gene of a two-gene operon, <italic>hpaT</italic>-<italic>hpaH</italic>, was non-pathogenic on barley and did not cause the HR or programmed cell death in non-host pepper similar to the <italic>hrcT</italic> mutant. The <italic>hpaT</italic> mutant phenotypes were partially complemented by either <italic>hpaT</italic> or the downstream gene, <italic>hpaH</italic>, which has been described as a facilitator of translocation in <italic>Xanthomonas oryzae</italic>. Interestingly, the <italic>hpaT</italic> mutant was also complemented by the <italic>hrpF</italic> gene from <italic>Xanthomonas euvesicatoria</italic>. These findings reveal that both HpaT and HpaH contribute to the injection of type III effectors into plant cells.</p>