Structural and functional heterogeneity of single-stranded DNA-binding proteins from calf thymus

A new purification technique for 'single-stranded DNA-binding proteins' from calf thymus permits the demonstration of a considerable heterogeneity within these proteins. Several molecular species are obtained with Mr between 24.10(3) and 30.10(3) and pI values between 6 and 8, showing significant differences with regard to the following functional properties: strength of binding to single-stranded DNA; lowering of melting temperature of poly[d(A-T)]; stimulation of DNA polymerase alpha on a poly[d(A-T)] template. Analysis of trypsin digestion products demonstrates that the different molecular species share extensive primary sequence homology. Experiments with antibodies show that the different molecular species are antigenically related and that a 31 kDa protein present in low amounts in our preparations is very cross-reactive.