ABSTRACT
We have analyzed transcripts encoding the variable regions of Ig heavy chains from adult and fetal bovine splenocytes and bovine x mouse heterohybridomas. The 13 adult, seven fetal and two heterohybridomas transcripts as well as the six genes that were sequenced had > 83% identity to each other in the VH-encoded regions (FRs 1-3 and CDRs 1 and 2). By this criterion, all the bovine sequences were assigned to one family, which corresponds to the bovine homolog of the murine Q52 family. Southern blot analysis of genomic DNA demonstrated that homologs of other murine VH families such as 7183, S107 and 36-60 were present in the genome, but transcripts from these families were not detected in rapid amplification of cDNA ends (RACE)-PCR amplified products or in individual clones. The sequences of the adult transcripts using the mu isotype showed extensive somatic mutation indicating that the process of somatic hypermutation begins earlier in development of the bovine B cell. The length of CDR3 from V(D)J rearrangements averaged 21 amino acids, which is larger than other mammalian CDR3s. Analysis of CDR3s from 23 fetal transcripts revealed a preference for a reading frame in the putative D genes which is rich in glycine and tyrosine, and is also extensively mutated in adults. The bovine immune system appears to utilize Ig VH genes of a single family, but generates antibody diversity by extensive somatic mutation and long CDR3s which are subsequently hypermutated.
Subject(s)
Immunoglobulin Heavy Chains/genetics , Immunoglobulin Variable Region/genetics , Multigene Family/immunology , Amino Acid Sequence , Animals , Base Sequence , Cattle , Gene Rearrangement, B-Lymphocyte, Heavy Chain/immunology , Immunoglobulin Heavy Chains/chemistry , Immunoglobulin Heavy Chains/isolation & purification , Immunoglobulin Joining Region/chemistry , Immunoglobulin Joining Region/genetics , Immunoglobulin Joining Region/isolation & purification , Immunoglobulin Variable Region/chemistry , Immunoglobulin Variable Region/isolation & purification , Mice , Molecular Sequence Data , MutationABSTRACT
Recent studies of the lifetime of fluorescence after picosecond pulse excitation of photosynthetic organisms revealed relatively complex decay kinetics that indicated a sum of three exponential components with lifetimes spanning the range from about 0.1-2.5 ns. These fluorescence lifetime data were examined in the context of a simple photochemical model for photosystem II that was used previously to account for fluorescence yield data obtained during continuous illumination. The model, which consists of a single fluorescing species of antenna chlorophyll and a reaction center, shows that, in general, the decay kinetics after pulse excitation should consist of the sum of two exponential decays. The model also shows that in going from open to closed reaction centers the lifetime of fluorescence may increase much more than the yield of fluorescence and surprisingly long fluorescence lifetimes can be obtained. However, conditions can be stated where fluorescence will decay essentially as a single component and with lifetime changes that are proportional to the yield changes. A heterogeneity was also introduced to distinguish photosystem II(alpha) units, which can transfer excitation energy among themselves but not the photosystem I, and photosystem II(beta) units, which can transfer energy to photosystem I but not to other photosystem II units. It is proposed that the rather complex fluorescence lifetime data can be accounted for in large part by the simple photochemical model with the alpha, beta heterogeneity in photosystem II.
