Fine Read­ing: The Good-Enough Clockus of Prochloro­coc­cus

by Elio

The very first post on this blog "It Don't Mean a Thing If It Ain't Got That Swing" was about the cir­ca­dian rhythm of cyanobac­te­ria. They turn their pho­to­syn­thetic ap­pa­ra­tus on and off to match the daily course of light and dark. This is a true clock-dri­ven cir­ca­dian rhythm be­cause it ma­nifests whether or not the light is on or off, thus is not just a re­sponse to a stim­u­lus.

What fas­ci­nated us then and con­tin­ues to be­daz­zle us now is that this rhythm has an un­der­stand­able bioche­mical ba­sis. Three "os­cil­la­tory" pro­teins are in­volved: KaiA, KaiB, and KaiC. To­gether, they lead to a cy­cle of phos­pho­ry­la­tion and de­phos­pho­ry­la­tion than not only sets the cel­lu­lar clock of these or­gan­isms, but that can be sim­u­lated in vitro with pu­ri­fied pro­teins! And, to chal­lenge our no­tion of what en­zymes do, these pro­teins do their en­zy­matic thing at such a slow pace that the mind bog­gles! One of them, KaiC, hy­drolyzes ATP at the light­ning speed of 15 mol­e­cules per day! (I've used up my al­lot­ment of ex­cla­ma­tion marks!)
 

A cyanobac­te­r­ial lineup. (left) Five di­verse Synecho­coc­cus strains. (right) Two Prochloro­coc­cus strains, the first one be­ing a sam­ple from the Mediter­ranean Sea and the sub­ject of the clock study. Cour­tesy of Ilka Ax­mann

Most of this won­der­land has been stud­ied in a strain of Syne­chococ­cus elon­ga­tus. A re­cent re­port from Ilka Ax­mann's lab in Berlin con­cerns the ma­rine cyanobacte­rium Prochloro­coccus, likely the most abun­dant photo­synthetic or­gan­ism there is. These bac­te­ria have KaiB and Kai C, but they have re­cently lost their gene for KaiA. Nev­er­the­less, they still ex­hibit some di­ur­nal pat­terns. But un­like the case with Syne­chococ­cus, if you put Prochlo­rococcus in con­tin­u­ous light, those pat­terns do not per­sist be­yond the cur­rent cy­cle. The au­thors liken the Prochloro­coc­cus sys­tem to an hour­glass, which can mea­sure a 24-hour pe­riod, but then has to be re­set by be­ing over­turned be­fore it can time the next day. Click here for a com­men­tary: The Rolex and the Hour­glass: a Sim­pli­fied Cir­ca­dian Clock in Prochloro­coccus?

The au­thors pro­pose that their data "pro­vide bio­chem­i­cal sup­port for the ob­served evo­lu­tion­ary re­duc­tion of the clock lo­cus in Prochloro­coc­cus and are con­sis­tent with a model in which a mech­a­nism that is less ro­bust than the well-char­ac­ter­ized Ka­iABC pro­tein clock of Synechococ­cus is suf­fi­cient for bi­o­log­i­cal tim­ing in the very sta­ble en­vi­ron­ment that Prochloro­coc­cus in­hab­its. This makes for an in­ter­est­ing vari­a­tion on the theme, ex­pand­ing the es­tab­lished no­tions about bi­o­log­i­cal clock­works.

Post­script: No sooner did we post this piece than the new is­sue of Mi­crobe (the mag­a­zine of the Amer­i­can So­ci­ety for Mi­cro­bi­ol­ogy) ar­rived, in­clud­ing a re­view by Carl Hirschie John­son en­ti­tled From Skep­ti­cism to Promi­nence: Cir­ca­dian Clocks in Bac­te­ria. We heartily rec­om­mend it. To ac­cess it, click here (link in­ac­tive).

Ref­er­ence

Ax­mann, I., Duhring, U., Seel­iger, L., Arnold, A., Vanselow, J., Kramer, A., & Wilde, A. (2009). Bio­chemical Ev­i­dence for a Tim­ing Mech­a­nism in Prochloro­coc­cus Jour­nal of Bac­te­ri­ol­ogy, 191 (17), 5342–5347. DOI 10.1128/JB.00419–09