Abstract
Small nuclear ribonucleoprotein particles U1 (U1 snRNPs) from HeLa cells were purified to homogeneity and examined in the electron microscope by negative staining with the double carbon film technique. A survey of several hundred views of the U1 snRNP particles revealed a main body about 8 nm in diameter with two characteristic protuberances 4-7 nm long and 3-4 nm wide. Anti-m3G antibodies were then used to define the site of the m3G base in the 5' cap of the U1 snRNA, close to the site where base-pairing occurs with the 5' splice sequence of pre-mRNA. The m3G base is located on the body of the U1 snRNP, close to one of the protuberances and approximately opposite the other.