The Ut­ter Magic of kDNA

Pic­tures Con­sid­ered #44

by Elio

Kine­to­plast DNAs (kDNA) are the most star­tlingly com­plex and un­ex­pected DNA struc­tures any­where. Super­latives do not suf­fice, as you will see. kD­NAs are found only in the kine­to­plast of fla­gel­lates, struc­tures in­volved in chro­mo­some seg­re­ga­tion, which is prob­a­bly the rea­son why they may not be as well known as they de­serve. In these or­gan­isms, mi­to­chon­dr­ial DNA is called kineto­plast DNA and con­sists of a net­work of thou­sands of in­terlocked (cate­nated) cir­cles that come in two sizes, maxi- (typ­i­cally 20 ‒ 40 kb) and mini-cir­cles (0.5 ‒ 1.0 kb). Usual­ly, maxis are present in only a few dozen per net­work, mi­nis in the thou­sands. To para­phrase a ter­rific re­view by Jensen and Eglund what do they do? "Maxi­cir­cles, like mi­tochondrial DNAs from more con­ven­tional or­gan­isms, en­code rRNAs and sev­eral pro­teins in­volved in en­ergy trans­duc­tion (in­clud­ing sub­units of cy­tochrome ox­i­dase, NADH de­hy­dro­ge­nase, and the ATP syn­thase). Maxi­cir­cle tran­scripts in  T. bru­cei are heav­ily edited, an amaz­ing re­action in which uridy­late residues are in­cor­po­rated into or re­moved from spe­cific in­ter­nal sites within the tran­script to cre­ate an open read­ing frame. Mini­cir­cles enco­de most of the small guide RNAs that are tem­plates for edit­ing speci­ficity. Be­cause the mi­to­chon­dr­ial tran­scripts in  T. bru­cei con­tain so many edit­ing sites, they re­quire many dif­fer­ent guide RNAs to pro­duce a set of func­tional mR­NAs. Thus, the mini­cir­cle pop­u­la­tion is large and hete­ro­geneous, with dif­fer­ent DNA species en­cod­ing dif­fer­ent guide RNAs. C. fas­ci­c­u­lata mini­cir­cles are less com­plex, with one pre­dom­i­nant mini­cir­cle class."

kDNA net­work struc­ture. A Elec­tron mi­cro­graph of the pe­riph­ery of an iso­lated kDNA net­work from T. avium. Loops rep­re­sent in­ter­locked mi­ni­circles (the ar­row­head in­di­cates a clear exam­ple). Bar, 500 nm. B Di­a­grams show­ing the orga­nization of mini­cir­cles. (I) Seg­ment of an isola­ted net­work show­ing in­terlocked mini­cir­cles in a pla­nar ar­ray. (II) Sec­tion through a con­densed net­work disk in vivo  show­ing stretched-out mi­ni­circles. The dou­ble-headed ar­row in­di­cates the thick­ness of the disk, which is about half the cir­cum­fer­ence of a mini­cir­cle. Source. Front­page: Elec­tron mi­cro­graph of a seg­ment of a kDNA net­work from C. fas­ci­c­u­lata. Small loops are mini­cir­cles. Source

This won­der­land de­serves at­ten­tion. As­suredly, read­ing about it will bring much en­joy­ment. For more on it, see the Jensen and En­glund re­view (Pay­wall) and an­other one here (Free Ac­cess link here).