Bovine seminal plasma contains a low-molecular-weight factor that inhibits RNA synthesis

A low-MW factor (800-1000 daltons) extracted from bovine seminal plasma (bSP) and partially purified by a five-step fractionation is very active in inhibiting RNA synthesis by E. coli RNA polymerase with calf thymus DNA as template (70% inhibition at factor:DNA ratio of about 1:100). The same factor also inhibits RNA synthesis in isolated liver nuclei but to a lesser extent. The bSP factor probably exerts its inhibitory activity on initiation rather than on the elongation processes. DNA heat denaturation experiments indicate that the factor stabilizes double-stranded DNA. The activity of bSP factor is almost destroyed by protease (pronase) digestion. Trypsin digestion is ineffective. Consequently, peptide integrity seems to be important for the biological activity. The factor is heat stable and does not contain nucleic acid components. Preliminary analysis indicates the presence of acidic amino acids with no basic or aromatic ones and that the active factor is not a product of histone or protamine degradation. When injected i.p. into 25-day-old female rats, the bSP factor has an inhibinlike activity.