The light-harvesting complex I (LHI) of Rhodobacter capsulatus is an oligomer of basic subunits each consisting of the two different pigment-binding polypeptides LHI a and LHI 0, encoded by the pufA (LHI a) and puJB (LHI j1) genes. Pulse-labeling experiments showed that in the presence of the LHI a polypeptide, the LHI , polypeptide was inserted earlier into the intracytoplasmic membrane than was the LHI a polypeptide.Each of the pufA and puJB genes was deleted to test whether the LHI a and polypeptides, respectively, are inserted into the intracytoplasmic membrane independently of the LHI partner polypeptide. Neither deletion mutant strain formed the LHI antenna, but a functional reaction center complex was present. Pulse-labeling experiments indicated that the LHI , polypeptide was inserted into the intracytoplasmic membrane with the same kinetics and in the same amounts regardless of whether the LHI a polypeptide was present. However, the LHI , polypeptide did not accumulate in the membrane in the absence of the LHI a protein but was degraded linearly within about 12 min. In contrast to the LHI ,1 protein, only trace amounts of the LHI a polypeptide were inserted into or attached to the membrane if the LHI , polypeptide was not synthesized.The facultative phototrophic bacterium Rhodobacter capsulatus has the ability to transduce light energy into metabolic energy. Two different light-harvesting (LH) complexes are involved in absorption of light quanta and transfer of excitation energy to the photosynthetic reaction center (RC). LH complex I (LHI) or B870 is associated with the RC forming the core complex (13, 21). LHII (B800-850) surrounds the core complex and is synthesized in variable amounts, depending on growth conditions (7, 13).The pigment-protein complexes are located in the intracytoplasmic membrane (ICM). Each complex is composed of two pigment-binding integral membrane proteins: LHI a and ,B, LHII a and 3, and RC-L and RC-M. A non-pigmentbinding protein is present in the LHII antenna (LHII -y polypeptide) and in the RC (RC-H subunit) (for reviews, see references 13 and 14). The pigments bacteriochlorophyll and carotenoids are bound noncovalently in stoichiometric ratios to the antenna and RC polypeptides (for a review, see reference 13).Pigments and pigment-binding proteins are coordinately assembled in distinct membrane areas (9) whose relative phospholipid concentration has an influence on the insertion (3,21,22,31,43 The LHI a and LHI fi polypeptides are encoded by the puf operon (1). Synthesis of these polypeptides is controlled at the mRNA level in response to changes of oxygen partial pressure or light intensity (1,5,22,50 hydrophobic a-helical central segment (39,41,51). The N-terminal segments of LHI a and LHI P polypeptides are oppositely charged (40, 42) and located on the cytoplasmic site of the membrane (41). The C termini of both LHI polypeptides extend into the periplasmic space (40). It was proposed that the LHI complex is stabilized by ionic interaction between the oppositely char...

1. The first example of a P450-dependent N-hydroxylation of an aminoguanidine (amidinohydrazone) is reported for 2-amino-5-chlorobenzophenone amidinohydrazone 1 (G 256) as substrate. 2. The N-hydroxylated metabolite 2 (2-amino-5-chlorobenzophenone N-hydroxyamidinohydrazone NOH-G256) and a further metabolite of 1, the phenol 3, were identified by tlc and ms analysis. 3. The microsomal reduction of an N-hydroxyaminoguanidine (N-hydroxy-amidino-hydrazone) was also demonstrated for the transformation of 2 to 1. 4. Both the N-hydroxylation of the aminoguanidine and the retroreduction of the N-hydroxyaminoguanidine were characterized by quantitative hplc analysis. 5. The conversion of the aminoguanidine 1 to N-hydroxyaminoguanidine 2 may be considered as an analogue of the physiological N-hydroxylation of arginine to N-hydroxyarginine by NO synthases.

The NH2 termini of light-harvesting complex I (LHI) polypeptides a and ,B of Rhodobacter caus are thought to be involved in the assembly of the LHI complex. For a more detailed study of the role of the NH2-terminal segment of the LHI a protein in insertion into the intracytoplasmic membrane (ICM) of R. ca,psulaus, amino acids 6 to 8, 9 to 11, 12 and 13, or The LHI a and 1 polypeptides are present in a 1:1 ratio. The LHI apoproteins are integral proteins of the intracytoplasmic membrane (ICM) and span the membrane once with * Corresponding author. a central hydrophobic domain (51-53). The NH2 terminus of the LHI a polypeptide is located in the cytoplasm and has a short hydrophobic central part of amino acid residues flanked by positively charged residues which are next to a conserved proline (51, 52). The LHI a protein is inserted into the ICM in wild-type amounts only in the presence of the LHI 1 protein (43). The LHI 1 protein, however, has a negatively charged NH2 terminus and is able to integrate temporarily into the membrane in the absence of the LHI a protein (43). The LHI 13 protein was observed to be inserted earlier into the ICM than was the LHI a protein (43).The described structures and features of the LHI a and 1 polypeptides are conserved among several species of the genus Rhodospirillaceae (50). Two highly conserved amino acid residues (Trp-8 and Pro-13) are present in the NH2 terminus of the LHI a polypeptide (50). The aromatic structure of the Trp at position 8 seems to be of importance for the insertion of the LHI a polypeptide into the ICM (42). The conservative amino acid residue Pro-13, however, seems to be necessary for keeping the LHI 1 partner polypeptide in the membrane (42).Deletions or insertions have been introduced into the NH2 terminus of the LHI a protein to obtain more information on the structural requirements for the membrane insertion of the LHI a protein and the assembly of both LHI proteins into a functional complex. The construction and characterization of R capsulatus LHI-strains containing a mutated LHI a polypeptide are described in this report. Additionally, LHI+ revertant strains of one LHI-mutant strain are characterized.3030

Trp‐8 and Pro‐13 of the Rhodobacter capsulatus light‐harvesting (LH) I α polypeptide are highly conserved among LHI and LHII α proteins of several species of the Rhodospirillaceae. Exchange of Trp‐8 and Pro‐13 to other amino acyl residues similar in structure and/or hydrophobicity indicates that Trp‐8 is involved in the insertion of the LHI α polypeptide into the intracytoplasmic membrane (ICM). Pro‐13, however, seems not to participate in the integration process of the LHI α protein but seems to be important for stable insertion of the LHI β partner protein in the ICM.