Glucose and trehalose PTS permeases of Spiroplasma citri probably share a single IIA domain, enabling the spiroplasma to adapt quickly to carbohydrate changes in its environment

Abstract

Spiroplasma citri is a plant-pathogenic mollicute phylogenetically related to Gram-positive bacteria. Spiroplasma cells are restricted to the phloem sieve tubes and are transmitted from plant to plant by the leafhopper vector Circulifer haematoceps. In the plant sieve tubes, S. citri grows on glucose and fructose, whereas in the leafhopper haemolymph the spiroplasma must grow on trehalose, the major sugar in insects. Previous studies in this laboratory have shown that fructose utilization was a key factor of spiroplasmal pathogenicity. To further study the implication of sugar metabolism in the interactions of S. citri with its plant host and its leafhopper vector, genes encoding permease enzymes II (EIIGlc and EIITre) of the S. citri phosphoenolpyruvate : glucose and phosphoenolpyruvate : trehalose phosphotransferase systems (PTS) were characterized. Mapping studies revealed that the EIIGlc complex was split into two distinct polypeptides, IIAGlc and IICBGlc, encoded by two separate genes, crr and ptsG, respectively. As expected, S. citri polypeptides IIAGlc and IICBGlc were more phylogenetically related to their counterparts from Gram-positive than to those from Gram-negative bacteria. The trehalose operon consisted of three genes treR, treP and treA, encoding a transcriptional regulator, the PTS permease (EIITre) and the amylase, respectively. However, in contrast to the fructose-PTS permease, which is encoded as a single polypeptide (IIABCFru) containing the three domains A, B and C, the trehalose-PTS permease (IIBCTre) lacks its own IIA domain. No trehalose-specific IIA could be identified in the spiroplasmal genome, suggesting that the IIBCTre permease probably functions with the IIAGlc domain. In agreement with this statement, yeast two-hybrid system experiments revealed that the IIAGlc domain interacted not only with IIBGlc but also with the IIBTre domain. The results are discussed with respect to the ability of the spiroplasma to adapt from the phloem sap of the host plant to the haemolymph and salivary gland cells of the insect vector.