RIBONUCLEIC ACIDS ASSOCIATED WITH MYOFIBRILS

In hearts of 16-day-old chick embryos, 85% of the ribonucleic acid (RNA) is of ribosomal type, i.e., 28 and 18 S; the remaining 15% is 4S RNA. The ribosomal ribonucleic acid (rRNA) is composed of 64% 28 S and 36% 18 S. The RNA isolated from myofibrils has a sedimentation profile similar to RNA from whole heart but only 5% of it is 4S RNA. This 4S RNA can be greatly reduced in quantity by washing the myofibrils. Myofibrillar RNA and rRNA have similar base compositions and identical ratios of 28S:18S components. The RNA isolated from nucleotropomyosin is composed of 28S and 18S components plus a small 4S peak. Ribo-nucleoprotein separated from myosin by high-speed centrifugation contains RNA which has sedimentation properties similar to rRNA. Ribosomes were isolated from highly purified myofibrils. Their sedimentation profiles and in vitro incorporation of [14C]phenylalanine into protein are the same as for microsomal ribosomes. Of the total RNA associated with myofibrils, 85% may be extracted as ribosomes.