Multiple CheY Homologs Control Swimming Reversals and Transient Pauses in Azospirillum brasilense

Abstract

Chemotaxis, together with motility, helps bacteria foraging in their habitat. Motile bacteria exhibit a variety of motility patterns, often controlled by chemotaxis, to promote dispersal. Motility in many bacteria is powered by a bidirectional flagellar motor. The flagellar motor has been known to briefly pause during rotation because of incomplete reversals or stator detachment. Transient pauses were previously observed in bacterial strains lacking CheY, and these events could not be explained by incomplete motor reversals or stator detachment. Here, we systematically analyzed swimming trajectories of various chemotaxis mutants of the monotrichous soil bacterium, Azospirillum brasilense. Like other polar flagellated bacterium, the main swimming pattern in A. brasilense is run and reverse. A. brasilense also uses run-pauses and putative run-reverse-flick-like swimming patterns, although these are rare events. A. brasilense mutant derivatives lacking the chemotaxis master histidine kinase, CheA4, or the central response regulator, CheY7, also showed transient pauses. Strikingly, the frequency of transient pauses increased dramatically in the absence of CheY4. Our findings collectively suggest that reversals and pauses are controlled through signaling by distinct CheY homologs, and thus are likely to be functionally important in the lifestyle of this soil organism.

Chemotaxis strategies of bacteria with multiple run modes

“…As soon as one or several motors change direction, the bundle falls apart and the cell undergoes a tumble event that results in an erratic change of the swimming direction ( 2 ). Species that carry only one flagellum (monotrichous flagellation) may interrupt their runs by pausings in the motor rotation, such as in the case of Rhodobacter sphaeroides ( 3 ), or they may change the sense of motor rotation to reverse their direction of motion and switch from a pushing to a pulling mode, such as many marine bacteria ( 4 ) or the soil bacterium Azospirillum brasilense ( 5 ).…”

Protein Activity Sensing in Bacteria in Regulating Metabolism and Motility

“…Flagellar speed is set at the rotary motor and its interacting flagellar basal body proteins (Nesper et al, 2017). The direction of flagella rotation is influenced by receptor proteins (methyl-accepting proteins [MCP]), which transmit signals to the flagellar base proteins (FliM) via the kinase CheA (Muok et al, 2019a) and the taxis response regulator protein CheY (Figure 1) (Baker et al, 2006;Di Paolo et al, 2016;Nishikino et al, 2018;Mukherjee et al, 2019;Ward et al, 2019). Therefore, signals feeding into flagellar rotational speed and direction intersect at the flagellar basal body.…”

Multiple CheY Proteins Control Surface-Associated Lifestyles of Azospirillum brasilense

“…The majority of motile, flagellated bacterial sequenced genomes indicates the presence of multiple chemotaxis as well as chemosensory (chemotaxis-like) pathways, with the latter displaying non-motility phenotypes such as extracellular matrix formation ( Edwards et al, 2011 ), cyst formation ( Berleman and Bauer, 2005 ; Wu et al, 2011 ), biofilm formation ( Huang et al, 2019 ), and quorum sensing ( Laganenka et al, 2016 ). In contrast to E.coli which possesses a single chemotaxis response regulator CheY to alter the direction of rotation of flagellar motors, the genome of many bacteria encodes for multiple CheY homologs: Rhodobacter sphaeroides ( Ferré et al, 2004 ; Porter et al, 2006 ), Sinorhizobium meliloti ( Schmitt, 2002 ), Rhizobium leguminosarum ( Miller et al, 2007 ), Azospirillum brasilense ( Mukherjee et al, 2016 , 2019 ), Borrelia burgdorferi ( Pitzer et al, 2011 ), Vibrio cholerae ( Hyakutake et al, 2005 ), etc. In some cases, the multiple CheY homologs are encoded within a single chemotaxis pathway (e.g., S. meliloti ).…”

“…A. brasilense cells are motile using a single polar flagellum that allows the cells to swim in liquid media and when the viscosity of the media increases, cells produce multiple lateral flagella, structurally distinct from the polar flagellum, that permit translocation across surfaces by swarming ( Moens et al, 1996 ). The polar flagellum of A. brasilense cells rotates in both clockwise and counterclockwise directions, and chemotaxis signaling controls the rotational bias of the polar flagellum in this species ( Zhulin and Armitage, 1993 ; Mukherjee et al, 2019 ). The A. brasilense polar flagellum is comprised of flagellin that is glycosylated ( Moens et al, 1995b ).…”

“…Genetic evidence and behavioral assays indicate that CheY6 activity is controlled by Che1/CheA1 signaling, and CheY7 activity, a mutant of which phenocopies a ΔcheA4 mutant, is controlled by Che4/CheA4 signaling ( Mukherjee et al, 2019 ; Figure 1A ). Mutants lacking cheA4 , cheY4, or cheY7 are unable of chemotaxis in spatial gradients of chemoeffectors, while mutants lacking cheY1 or cheY6 still display chemotaxis under these conditions ( Bible et al, 2008 ; Mukherjee et al, 2016 , 2019 ). Another feature of the polar flagellum motor of A. brasilense is that it undergoes brief swimming pauses, which are distinct from speed increases or reversals.…”