283 Biochimica et Biophysica Acta, 374 (1974) 283--291 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands BBA 98176 SIZE AND CONFIGURATION OF MITOCHONDRIAL DNA IN EUGLJENA GRA CILIS MARGIT M.K. NASS a, LEA SCHORI b, YEHUDA BEN-SHAUL b and MARVIN EDELMAN c aDepartment of Therapeutic Research, University of Pennsylvania School of Medicine, Philadelphia, Pa. 19104 (U.S.A.), b Laboratory for Electron Microscopy, Tel A viv University, Tel-Aviv and CDepartment of Plant Genetics, The Weizmann Institute of Science, Rehovot (Israel) (Received May 27th, 1974) Summary (1) The size and structure of mitochondrial DNA from Euglena gracilis (permanently bleached strain) were analyzed by a variety of different isolation and analytical methods. In all cases in which endogenous nuclease activity and shearing of DNA were minimized, small molecules with a mean length of 0.9--1.0/~m (Mr = 1.9 • 106 --2.1 • 106 ) were found. (2) The DNA was linear under most conditions, but a small percentage of circular molecules (1.01 -+ 0.05 /~m) was obtained from mitochondrial frac- tions, especially after pre-extraction of cells with 80% acetone. The DNA banded at the buoyant density typical of mitochondrial DNA. Membrane-asso- ciated DNA molecules which appeared to be circular (0.95 -+ 0.05 pro) and linear forms (0.96 + 0.03 /~m) were released from osmotically shocked mito- chondria. (3) We conclude that Euglena mitochondrial DNA is about 1 pm in length, considerably smaller in size than other known animal and plant mitochondrial DNA. The possibility is discussed that Euglena mitochondrial DNA in situ exists in both circular and linear form. Introduction The sizes of mitochondrial DNAs are of particular interest with respect to their potential informational role within the organism. In contrast to mitochon- drial DNAs from a wide variety of animal species which contain 4.45--5.2 pm circular molecules [1--3], a spectrum of sizes and structures has been reported for mtDNAs of some lower organisms and plants (cf. refs 2,3). These range from 0.3--0.8 pm circular molecules in kinetoplasts of several genera of Try-