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Phenylalanyl-tRNA synthetases [L-phenylal-anine:tRNAPb ligase (AMP-forming), EC 6.1. An ac22 structure was proposed, too, for the yeast mitochondrial phenylalanyl-tRNA synthetase, which has a molecular weight and subunit sizes very close to those of its cytoplasmic counterpart (17). Recently, the gene MSFI encoding the mitochondrial a subunit was isolated and sequenced (18). Despite considerable sequence divergence between the mitochondrial and cytoplasmic phenylalanyltRNA synthetases as far as the a subunit is concerned, their striking similarity in size was taken into consideration to examine the possibility that the mitochondrial a subunit could combine with the cytoplasmic ,B subunit to build up a functional chimera. To test this idea, we created an operon with the corresponding MSFI and FRSI structural genes, using the E. coli pheST intergenic region (15) to produce a polycistronic mRNA. A construction with the MSFI gene alone fused to the lacZ gene was used as a control, assuming that an isolated mitochondrial subunit would be inactive, as is the case for individual subunits of the yeast and E. coli phenylalanine enzymes. To our surprise the MSF1 protein by itself was found to be active. We purified the MSF1 protein and measured its catalytic properties. Turnover rates in the aminoacylation reaction were found to be comparable to those of monomeric or oligomeric aminoacyl-tRNA synthetases. Similarly, the same fully active monomeric protein was obtained when the MSF1 protein was expressed from the MSFI-FRSJ operon, indicating that no heterologous association of the mitochondrial and cytoplasmic subunits had occurred. Therefore, we focused our attention on the quaternary structure and tRNA specificity of the MSF1 protein; the data reported here support the model of a monomeric mitochondrial phenylalanyl-tRNA synthetase.MATERIALS AND METHODS lacZ-MSFI Gene Fusion. MSFJ was fused to lacZ by using procedures and vectors similar to those used in the construction of the phenylalanyl-tRNA synthetase operon (19). Plasmid pG120/ST10 (18) was the source of the MSFI structural gene. An Xba I restriction site in the MSFI gene was created by using oligonucleotide AACATTGAAGTTACCTCCAT-*Present address: Universite Nationale du Benin, Ddpartement de Biochimie, B.P. 526, Cotonou, Benin. tTo whom reprint requests should be addressed. *In our previous work, a and ( designated the large and small subunits of yeast phenylalanyl-tRNA synthetase, respectively. To comply with the rule adopted for the other cited aminoacyl-tRNA synthetase subunits, we have changed the order of designation: a, small, and,8, large. 8387The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

The complete sequence (153 amino acids) of horse heart myoglobin has been established. The sequence of a peptide isolated from chymotrypsin hydrolyzates of globin allowed to fill a gap in the tentative sequence previously proposed; this peptide was studied by means of pepsin hydrolysis, action of N-bromosuccinimide on histidyl bonds, and Edman degradation method associated with dansylation.After hydrolysis of globin by chymotrypsin treated by TLCK, the same major peptides were found as after hydrolysis by non-treated chymotrypsin. These peptides were identified by ionexchange resins and bidimensional paper chromatography, and also by the determination of their amino-acid composition.The suppression of secondary cuts explains the presence of new peptides, whose the composition and N-terminal sequences were determined. It was then possible t o confirm several sequences which could be previously deduced from the study of split products after cyanogen bromide treatment. The specificity of chymotrypsin towards certain types of peptide bonds is discussed.Horse myoglobin, as compared with sperm whale myoglobin, contains 18 differences : 17 substitutions and one inversion. The major part of the substitutions are located in N-and C-terminal sequences; they are all punctiform, resulting from the replacement of only one base in each codon.Dans des memoires anterieurs [l,2], nous avons ktudii: les peptides libisrks par hydrolyse chymotrypsique de la globine de myoglobine du cceur de cheval. Nous avons egalement publie [3--51 les sequences en amino-acides des trois segments 1ibi:rbs par l'action du bromure de cyanogbne sur la globine. Ces rbsultats nous avaient dbjB permis de proposer [6] une structure partielle de la myoglobine de cheval. La sequence publiite prhsentait un certain nombre de lacunes qu'il s'agissait de combler : certaines incertitudes ont dejB 6tk levees par 1'Btude [Z] des peptides lib6ri.s par action de la chymotrypsine (enchainements 38-39, 52-53, 66-67, 102-113 et 125-129); il en est de m6me de l'enchainement Lys-Lys-Lys-en position 77-78-79 en faveur duquel nous ne posskdions que des arguments indirects [5]: 1'6tude de l'hydrolysat par la chymotrypsine du produit de guanidylation de la globine nous a rhcemment apporti: des preuves directes [7].Nous prbcisons dans ce memoire l'enchainement des rhsidus 18 B 28 par l'htude du peptide chymotrypsique SZElSlP3. L'hydrolyse de la globine par Abbrkviations non usuelles. TLCK, tosyl-lysine-chloromethyl-ketone ou l-chloro-3-tosylamido-7-amino-2-heptanone.Enzymes. Trypsine (EC 3.4.4.4) ; chymotrypsine (EC 3.4.4.6) ; carboxypeptidase A (EC 3.4.2.1); leucine-aminopeptidase (EC 3.4.1.1).

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