# 7.1D: Slipped-Strand Mispairing

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Slipped strand mispairing (SSM) is a process that produces mispairing of short repeat sequences during DNA synthesis.

## LEARNING OBJECTIVES

Explain how slipped-strand mispairing can be used as a mechanism to regulate gene expression

## Key Points

• Altered gene expression is a result of SSM and depending where the increase or decrease of the short repeat sequences occurs in relation to the promoter will either regulate at the level of transcription or translation. The outcome is an ON or OFF phase of a gene or genes.
• SSM can result in an increase or decrease in the number of short repeat sequences. The short repeat sequences are 1 to 7 nucleotides and can be homogeneous or heterogeneous repetitive DNA sequences.
• Transcriptional regulation can occur if the repeats are located in the promoter region at the RNA polymerase binding site, -10 and -35 upstream of the gene(s).
• SSM induces transcriptional regulation is by changing the short repeat sequences located outside the promoter. If there is a change in the short repeat sequence it can affect the binding of a regulatory protein, such as an activator or repressor.

## Key Terms

• Slipped strand mispairing: a process that produces mispairing off short repeat sequences between the mother and daughter strand during DNA synthesis.

Slipped strand mispairing (SSM) is a process that produces mispairing of short repeat sequences between the mother and daughter strand during DNA synthesis. This RecA-independent mechanism can transpire during either DNA replication or DNA repair and can be on the leading or lagging strand and can result in an increase or decrease in the number of short repeat sequences. The short repeat sequences are 1 to 7 nucleotides and can be homogeneous or heterogeneous repetitive DNA sequences.

Altered gene expression is a result of SSM and depending where the increase or decrease of the short repeat sequences occurs in relation to the promoter will either regulate at the level of transcription or translation. The outcome is an ON or OFF phase of a gene or genes.

Transcriptional regulation occurs in several ways. One possibility is if the repeats are located in the promoter region at the RNA polymerase binding site, -10 and -35, upstream of the gene(s). The opportunistic pathogen H. influenzae has two divergently oriented promoters in fimbriae geneshifA and hifB. The overlapping promoter regions have repeats of the dinucleotide TA in the -10 and -35 sequences. Through SSM the TA repeat region can undergo addition or subtraction of TA dinucleotides which results in the reversible ON phase or OFF phase of transcription of the hifA and hifB. The second way that SSM induces transcriptional regulation is by changing the short repeat sequences located outside the promoter. If there is a change in the short repeat sequence, it can affect the binding of a regulatory protein, such as an activator or repressor. It can also lead to differences in post-transcriptional stability of mRNA.