a, SDHB-1 is the iron–sulphur subunit of complex II. The promoter and mitochondrial localization sequence of SDHB-1 was used for targeted expression of cpYFP in the mitochondrial matrix. SDHA, B, C and D, succinate dehydrogenase subunit A, B, C and D; Q, coenzyme Q. b, Low-magnification images of a C. elegans worm with an integrated Psdhb-1::mtLS::cpYFP transgene at a long (left) or short (right) exposure time. The pharynx has the highest expression level of mt-cpYFP. c, High-magnification images showing that mt-cpYFP co-localized with mito-tracker red in the pharynx, body-wall muscles, intestine and germ cells. The mitoflash activity in the pharynx (about one to four mitoflash events per anterior pharynx per 200 s; see Fig. 1e and the WT traces in Fig. 2) is at least tenfold that in the body-wall muscles (the average mitoflash events per cell per 200 s on adult days 1, 3, 5 and 9 were 0.1, 0.1, 0.1 and 0.2, respectively; n = 16, 16, 12 and 12) or the intestine (less than 0.1 mitoflash event per cell per 200 s on adult day 3; n = 40).