Synaptonemal complexes (SCs) are structures that are formed between homologous chromosomes during meiotic prophase. They are probably involved in chromosome pairing and recombination. Using a monoclonal anti‐SC antibody we isolated cDNAs encoding a major component of SCs which is localized specifically in synapsed segments of meiotic prophase chromosomes. The protein predicted from the nucleotide sequence of a full‐length cDNA, named SCP1, consists of 946 amino acid residues and has a molecular weight of 111 kDa. It shares several features with nuclear lamins and some recently identified nuclear matrix proteins. The major part of SCP1 consists of long stretches capable of forming amphipathic alpha‐helices. This region shows amino acid sequence similarity to the coiled‐coil region of myosin heavy chain. A leucine zipper is included in this region. The carboxy‐terminus has two small basic domains and several S/T‐P‐X‐X motifs, which are characteristic of DNA‐binding proteins. One of these motifs is a potential target site for p34cdc2 protein kinase. The amino‐terminus is acidic and relatively proline‐rich, but does not contain the S/T‐P‐X‐X motif. The transcription of the gene encoding SCP1 is restricted to zygotene‐diplotene spermatocytes. A polyclonal antiserum raised against the fusion protein of one of the cDNA clones recognizes a single protein on Western blots of isolated SCs, with an electrophoretic mobility identical to that of the antigen recognized by the original monoclonal antibody (mAb), IX5B2. From a detailed comparison of the immunogold labelling of rat SCs by mAb IX5B2 and the polyclonal anti‐fusion protein antiserum respectively, we tentatively infer that the carboxy‐terminus of SCP1 is orientated towards the lateral elements and that the other domains of the protein extend towards the central region between the lateral elements. We conclude that SCP1 is the major component of the transverse filaments of SCs, and speculate that it has evolved by specialization of a nuclear matrix protein.

The time-resolved fluorescence characteristics of flavin in oxidized flavodoxin isolated from the anaerobic bacterium Clostridium beijerinckii have been examined. The fluorescence intensity decays were analyzed using the maximum-entropy method. It is demonstrated that there exist large differences in fluorescence behaviour between free and protein-bound FMN. Three fluorescence lifetime components are found in oxidized flavodoxin, two of which are not present in the fluorescenceintensity decay of free FMN. The main component is distributed at 30 ps, with relative contribution of 90%. Another minor component (4% contribution) is distributed at 0.5 ns. The third component is distributed at 4.8 ns (6%), coinciding with the main distribution present in the fluorescence decay of free FMN. The results allowed us to determine the dissociation constant, Kd = 2.61 X 10--'"M (at 20°C).Collisional fluorescence-quenching experiments revealed that the flavin moiety responsible for the longest fluorescence lifetime is, at least partially, exposed to the solvent. The shortest lifetime is not affected significantly, indicating that it possibly originates from an active-site conformation in which the flavin is more or less buried in the protein and not accessible to iodide.The fluorescence anisotropy behaviour of free and protein-bound FMN was examined and analyzed with the maximum-entropy method. It was found that an excess of apoflavodoxin is required to detect differences between free and protein-bound FMN. In free FMN one single distribution of rotational correlation times is detected, whereas in flavodoxin the anisotropy decay is composed of more than one distribution.Associative analysis of fluorescence anisotropy decays shows that part of the 4.8 ns fluorescence lifetime present in the flavodoxin fluorescence decay, is coupled to a rotational correlation time similar to that of free FMN in solution, while another part of this lifetime is coupled to a longer correlation time of about 1 ns. This finding is in accordance with earlier studies [Barman, B. G. & Tollin, G. (1972) Biochemistry 11, 4746-47541 in which it was proposed that the first binding step of the flavin to the protein involves the phosphate group rather than another part of the FMN.The two shortest fluorescence lifetimes, which do not carry information on the long-term rotational behaviour of the protein, seem nonetheless to be associated with a longer rotational correlation time which is comparable to overall protein tumbling. These lifetime components probably originate from a complex in which the flavin-ring system is more or less immobilized within the protein matrix.Flavodoxins are low-molecular-mass electron-transport proteins (14-25 kDa) which can substitute for ferredoxin in a number of reactions (Mayhew, 1971 ;Mayhew and Ludwig, 1975;Mayhew and Tollin, 1992). Flavodoxins can be isolated from some algae and several anaerobic and aerobic bacteria grown under iron-containing or iron-deficient conditions (Knight and Hardy, 1966;Benemann et al., 1969;Mayh...