The structural gene for cytochrome c 2 ( cycA ) of the photosynthetic bacterium Rhodopseudomonas capsulata has been cloned, and the nucleotide and deduced polypeptide sequences have been determined. Compared with the known amino acid sequence of the purified cytochrome c 2 , the nucleotide sequence corresponding to the N-terminal part of the cycA gene product indicates the presence of a putative 21 amino acid signal sequence. Thus, cytochrome c 2 may be synthesized as a precursor which is processed during its secretion to the periplasm. Insertion and insertion-deletion mutations were constructed in vitro and the chromosomal cycA + allele of a wild-type strain was replaced with these mutations by homologous recombination to yield c 2 - mutants of R. capsulata . The c 2 - mutants are stable, and they can grow by photosynthesis and by respiration. Since cytochrome c 2 is the primary electron donor to the reaction center during photosynthesis, the ability of these mutants to grow photosynthetically indicates that an alternative way(s) of reducing the oxidized reaction center must exist in R. capsulata . One candidate for this role may be the membrane-bound cytochrome c 1 .

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We have recently shown that, after the histones and most of the nonhistone proteins are gently removed from HeLa metaphase chromosomes, the chromosomal DNA is still highly organized and relatively compact. The structure of these histone-depleted chromosomes is due to the presence of a number of nonhistone proteins that form a central scaffold that retains the approximate size and shape of intact chromosomes and to which the DNA is attached, predominantly forming loops. We now demonstrate that the protein scaffold may be iso ated independently of the DNA by treating HeLa chromosomes with micrococcal nuclease before removing the histones.The chromosomal scaffolds may be isolated by sucrose density gradient centrifugation as a well-defined peak that is stable in 2 M sodium chloride, but is dissociated by treatment with proteases, 4 M urea, or 0.1% sodium dodecyl sulfate. Polyacrylamide gel electrophoresis reveals that the protein content of scaffold preparations is identical to that of histone-depleted chromosomes. Fluorescence microscopy of purified scaffolds in isolation buffer shows that the particles still possess the familiar chromosome morphology. When the scaffolds are examined in the electron microscope, a fibrous structure with the approximate size and shape of intact, paired chromatids is seen. Less than 0.1I% of the chromosomal DNA and virtually no histones are associated with the purified scaffold structures. Mitotic chromosomes are composed of the five histones and a large number of nonhistone proteins that keep the DNA in a highly compact arrangement. Electron microscopy has revealed that the nucleoprotein fiber in mitotic chromosomes is a "knobby" fiber with a diameter of about 200-300 A and appears in a complex, net-like arrangement (e.g., ref. 1). The complexity of this network and the tendency of the chromatin fibers to adhere to each other make it difficult to study the higher-order folding of the nucleoprotein fiber in chromosomes.A great deal has been learned in the last few years about the structural role played by the histones in chromatin, but little is known about a possible involvement of nonhistone proteins in chromosome structure. Starting with the hypothesis that nonhistone proteins-may play a role in determining the superstructure of interphase and metaphase chromosomes, we have recently developed an approach that permits dissection of the structural contribution made by the histones and certain nonhistone proteins. Our general approach toward uncovering a structural role for the nonhistone proteins has been to gently remove the histones from chromosomes by competition with dextran sulfate and heparin, without destroying the continuity of the DNA fiber (2). In this way, all the histones and many nonhistone proteins are removed, leaving behind some tightly bound nonhistone proteins that keep the chromosomal DNA highly organized and compact. This dissecting procedure hasThe costs of publication of this article were defrayed in part by the payment of page charges. This article mu...

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