Monoclonal antibodies for the human insulin receptor were produced following immunization of mice with IM-9 lymphocytes and/or purified placental receptor. Four separate fusions yielded 28 antibodies, all of which reacted with receptor from human placenta, liver and IM-9 cells. Some antibodies cross-reacted to varying degrees with receptor from rabbit, cow, pig and sheep, but none reacted with rat receptor. At least 10 distinct epitopes were recognized as indicated by species specificity and binding competition experiments. All of these epitopes appeared to be on extracellular domains of the receptor as shown by binding of antibodies to intact cells. In some cases the epitopes were further localized to alpha or beta subunits by immunoblotting. Several antibodies inhibited binding of 125I-insulin to the receptor, some had no effect on binding, and others enhanced the binding of 125I-insulin. It is concluded that these antibodies will be valuable probes of receptor structure and function.

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Sequences at the immediate 3′ terminus of several eukaryotic primary transcripts, synthesised just before the termination of transcription, are often lost during RNA processing. The rna82.1 mutation in Saccharomyces cerevisiae appears to result in a deficiency of the endonuclease that removes such sequences from certain yeast transcripts. Some small RNAs of rna82.1 cells are a few nucleotides longer than their counterparts in wild‐type S. cerevisiae. The 5S rRNAs made during very short pulse‐labellings of the mutant have, relative to the mature 121 nucleotide 5S RNA of wild‐type cells, an additional 7, 11 or 13 nucleotides at their 3′ terminus. These 5S forms reveal sites upon 5S genes where transcription probably terminates in vivo. The extra nucleotides upon 5S RNAs in rna82.1 cells are lost very slowly by sequential removal from the 3′ terminus. Through this 3′‐5′ exonuclease action the total 5S RNA of the mutant possesses several 3′‐terminal sequences yet is mostly only 0‐3 nucleotides longer than in wild‐type S. cerevisiae. Just one or two of these 3′‐terminal sequences serve as a substrate in vivo for a poly(A) polymerase since a small proportion of rna82.1 5S RNAs terminate in the sequence: CAAUCUUU(A)n.

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