Three sequences in the vicinity of poly (A) addition sites are conserved among vertebrate mRNAs. We analyze the effects of single base changes in each position of AAUAAA and in the nucleotide to which poly (A) is added on 3' end formation in vitro. All 18 possible single base changes of the AAUAAA sequence greatly reduce addition of poly (A) to RNAs that end at the poly (A) addition site, and prevent cleavage of RNAs that extend beyond. The magnitude of reduction varies greatly with the position changed and the base introduced. For any given mutation, cleavage and polyadenylation are reduced to similar extents, strongly suggesting that the same factor interacts with AAUAAA in both reactions. Mutations at and near the conserved adenosine to which poly (A) is added disturb the accuracy, but not the efficiency, of 3' end formation. For example, point mutations at the conserved adenosine shift the 3' end of the most abundant 5' half-molecule downstream by a single nucleotide. The mechanism by which these mutations might exert their effects on the precision of 3' end formation are discussed.

authors request that the following corrections be noted. It was accidentally stated that the studies by Kajita et al. (1) and Lee et al. (2) dealt with cinnamoyl-CoA reductase modified plants when in fact they concerned 4-coumarate:coenzyme A ligase (4CL) transgenic plants. Lignin concentration was reduced by down-regulation of 4CL activity in both studies (1, 2). In a subsequent article, Kajita et al. (3) reported a negligible decrease in lignin concentration and a decreased syringyl-toguaiacyl ratio for lignin composition of a sense-suppressed 4CL transgenic tobacco line. Kajita et al. (1) rather than Kajita et al. (3) was inadvertently cited when this later report was contrasted with the large decreases in lignin concentration and an increased syringyl-to-guaiacyl lignin ratio for anti-sense suppressed 4CL Arabidopsis transgenics (2). The authors apologize for the confusion these errors have created for readers of their Commentary and to the authors of the cited work for misrepresenting their research. November 10, 1998, of Proc. Natl. Acad. Sci. USA (95, 13612-13617), the authors request that the following correction be noted: In Fig. 2 appearing on page 13614, the genotype identification for testicular histology in panels C and D were shown reversed. The correct identification is Ϫ͞Ϫ for panel C and ϩ͞ϩ for panel D. The fifth sentence of the figure legend should read as follows: "Histological sections at lower (E) and higher (D) magnification of the seminiferous tubuli from a wild-type and mutant (F and C) mouse."Cell Biology. In the article "Efficient construction of a large nonimmune phage antibody library: The production of highaffinity human single-chain antibodies to protein antigens" by