Variable length tandem repeat polyglutamine sequences in the flexible tether region of the Tsr chemotaxis receptor of Escherichia coli

Abstract

Methyl-accepting chemotaxis proteins (MCPs) are receptors that play an important role in bacterial chemotaxis. Methylation of Tsr, the MCP that mediates chemotaxis towards serine in Escherichia coli, is thought to be facilitated by binding of the methyltransferase to a flexible tether region at the C-terminal end of Tsr. This study analysed natural length variants of the tether that occur in E. coli due to genetic instability in tandem repeat DNA sequences that code for glutaminyl (Q) residues, creating polyQ sequences of variable lengths in the tether region. The tsr gene of E. coli K-12 (strain MG1655) codes for 4Q at the beginning of its 35 aa tether region. The tether varies in length from 35 to 47 residues among pathogenic and non-pathogenic strains of Escherichia, Shigella spp., Salmonella, Yersinia and Photorhabdus. Among previous sequences, Escherichia and Shigella mostly have 4Q and 7Q variants, and one strain (E. coli HS) has 10Q. In E. coli isolated from 50 humans and 75 animals (dogs, cats, horses, birds, etc.), polyQ up to 13Q (44 aa tether) were identified (6 strains); relative frequencies were 7Q (~77 % of the total) .4Q (14 %) .13Q (5 %) .10Q (4 %). Phylogenetic analysis revealed that E. coli strains with 10Q or 13Q largely fell within two clusters. Serine chemotaxis was not significantly different among 7Q, 10Q and 13Q strains, and was comparable to chemotaxis in the frequently studied K-12 strain. These results are consistent with models indicating that polyQ sequences from 7Q to13Q are flexible, and that longer tethers, within this range, would not change the precision of adaptation mediated by methylation. Studies of this naturally variable polyQ region in E. coli may also have relevance to mechanisms that mediate polyQ instability in human genetic diseases. INTRODUCTIONThe receptors that mediate one form of chemotactic behaviour in bacteria have been an important focus of study since Adler and colleagues first described them in Escherichia coli (Adler, 1969;Springer et al., 1979). E. coli generally have five different types of receptors (Tar, Tsr, Trg, Tap and Aer) that differ in their sensitivities to different attractants and repellents, and form mixed clusters of homodimers (Studdert & Parkinson, 2005) at the cell poles (Maddock & Shapiro, 1993) in proportion to their relative abundance in the cells (Studdert & Parkinson, 2004;Gosink et al., 2006). As reviewed recently (Hazelbauer et al., 2008), binding of an attractant chemical to the periplasmic sensing domain of a chemoreceptor molecule decreases the activity of a histidine kinase (CheA) coupled, via the CheW protein, to its cytoplasmic signalling domain. Decreased CheA activity lowers the phosphorylation state of the flagellar motor regulator, CheY, thereby increasing the probability of counterclockwise flagellar rotation, which mediates forward swimming. The signalling properties of the chemoreceptors, except for Aer, are also regulated by methylation and demethylation of several specific glutamyl residues in their cytoplasmic domain...