The twin‐arginine translocation (Tat) system is essential for Rhizobium–legume symbiosis

Abstract

SummaryThe Tat (twin-arginine translocation) system mediates export of periplasmic proteins in folded conformation. Proteins transported via Tat contain a characteristic twin-arginine motif in their signal peptide. Genetic determinants ( tatABC genes) of the Tat system from Rhizobium leguminosarum bv. viciae were cloned and characterized, and a tatBC deletion mutant was constructed. The mutant lacked the ability for membrane targeting of hydrogenase, a known Tat substrate, and was impaired in hydrogenase activity. Interestingly, in the absence of a functional Tat system, only small, white nodules unable to fix nitrogen were induced in symbiosis with pea plants. Analysis of nodule structure and location of green fluorescent protein (GFP)-tagged bacteria within nodules indicated that the symbiotic process was blocked in the tat mutant at a stage previous to bacteria release into cortical cells. The R. leguminosarum Tat-deficient mutant lacked a functional cytochrome bc 1 complex. This was consistent with the fact that R. leguminosarum Rieske protein, a key component of the symbiosis-essential cytochrome bc 1 complex, contained a typical twin-arginine signal peptide. However, comparative analyses of nodule structure indicated that nodule development in the tat mutant was arrested at an earlier step than in a cytochrome bc 1 mutant. These data indicate that the Tat pathway is also critical for proteins relevant to the initial stages of the symbiotic process.

Gene Products of the hupGHIJ Operon Are Involved in Maturation of the Iron-Sulfur Subunit of the [NiFe] Hydrogenase from Rhizobium leguminosarum bv. viciae

“…The subcellular localization of structural subunits was investigated in the hupH mutant with UPM1155(pALPF1) and SM61(pALPF1) as controls. SM61 is a tatBC mutant affected in hydrogenase translocation to the cytoplasmic membrane (25). In the mutant strain containing the hupH deletion, HupL was located exclusively in the soluble fraction (Fig.…”

“…These results are specially relevant since, apart from the structural component and the Tat system (25), no other proteins with a role in synthesis of a functional iron-sulfur subunit of [NiFe] hydrogenase have been identified.…”

Role of the Twin Arginine Protein Transport Pathway in the Assembly of the Streptomyces coelicolor Cytochrome bc 1 Complex

“…In contrast, in bacteria and plants, the Rieske iron-sulfur protein is inserted into the membrane by the Tat machinery (17,18,19,20,21). The Tat pathway transports folded substrate proteins (see references 22 and 23 for recent reviews), and the iron-sulfur cluster is inserted into the Rieske protein in the cytoplasm prior to transport.…”

The Twin Arginine Transport System Appears To Be Essential for Viability in Sinorhizobium meliloti

“…The majority of these appear to be expressed or function anaerobically (43). Interestingly, bioinformatic analysis of S. meliloti and Rhizobium leguminosarum suggests that a much larger number of proteins may use the Tat system in these organisms (36). In addition, these organisms are classified as obligate aerobic organisms (12,28,55).…”

“…Since tat mutations have been shown to affect many bacteriahost interactions (17,25,36,49,62), we set out to construct a tat mutation in S. meliloti to elucidate the role that tat may have in determining the bacteria's ability to interact with its host plant and affect nodule development. Moreover, we reasoned that a tat mutation in S. meliloti might help characterize putative Tat substrates in a different model organism.…”