Strand bias in Ig somatic hypermutation is determined by signal sequence within the variable region

Ig genes undergo hypermutation with a nucleotide preference of A over T for mutation on the coding strand. As only with concomitant strand bias can su ch nucleotide bias be observed, Ig gene hypermutation is generally accepted as a strand-specific process, for which the mechanistic basis remains unkn own, It has previously been shown that different non-Ig sequences replacing the LVJ region of an Ig transgene to various extents are targeted for hype rmutation with similar mutation frequencies. However, the nucleotide bias c haracteristic of Ig hypermutation was not found in two of the three such se quences studied. To test whether it is the DNA sequences of the non-Ig subs trates that determine the pattern of nucleotide bias in hypermutation or wh ether the LVJ sequence may contain element(s) that confer strand bias, we h ave added back all the replaced LVJ sequences to one of the transgenes, L-k -Vgpt*, that expresses no strand bias in hypermutation and studied the outc ome, The results show that the gpt sequence in the presence of the complete LVJ sequence hypermutates differently from the same sequence in L-k-Vgpt* where 84% of the LVJ was replaced, The main difference is the resumption of strand bias characteristic of Ig hypermutation. Thus, whether or not a sub strate sequence manifests strand bias in hypermutation is not inherently de termined by the substrate DNA sequence. This indicates the presence of spec ial element(s) within the LVJ that confer strand bias.