A STRONG PROPENSITY TOWARD LOOP FORMATION CHARACTERIZES THE EXPRESSED READING FRAMES OF THE D-SEGMENTS AT THE IG H AND T-CELL RECEPTOR LOCI

A compilation of murine and human Ig H and TcR beta-D segment sequences was used to estimate the relative usage of the various reading frames and to look for associated sequence patterns. We confirm a strong bias in the expression of the Ig H D segments, with more than 90% (murine) and 85% (human) expressed peptides resulting from a preferred reading frame. Remarkably, 86% (mouse) and 90% (human) of those peptides contain at least one glycine residue. All but one of the atypical preferred D peptides contain serine or proline residues and are found in the immediate vicinity of glycine residues provided by specific J(H) segments. The presence of tyrosine residues is also a characteristic feature of expressed reading frames in both mouse (75%) and human (90%). These results suggest that the constraints of forming a flexible loop within the third complementarity-determining region, is a factor in the preference for a particular reading frame in Ig H D. For the TcR beta-D segments, glycine is specified in most reading frames, and no significant preference is observed.