Cloning of cDNAs coding for human HMG I and HMG Y proteins: both are capable of binding to the octamer sequence motif.

In human B lymphocytes and placenta HMG I and its smaller isoform HMG Y are encoded by two distinct but structurally highly similar mRNAs which arise most likely by alternative splicing of a single primary transcript. Both have been cloned as cDNAs. On Northern blots an abundant mRNA species 2000 nucleotides in length was detected in all cell lines examined. Exclusively in erythroid cells an additional rare 3800 nucleotides long mRNA species was noted. In quiescent cells the mRNA levels of HMG I/Y were not significantly down-regulated. Southern blot analysis indicated that at least four genes are present per haploid human genome. Both proteins when expressed in bacteria bind specifically to A-T rich stretches of DNA suggesting that no posttranslational modifications are necessary for specific DNA binding. Interestingly, HMG I as well as HMG Y are capable of binding to the octamer transcriptional regulatory sequence motif.