Final answer:
The step not part of the SOS repair is 'C. All targets of RecA are cleaved at the dipeptide Ala-Gly' as this is not accurate; RecA specifically induces the self-cleavage of LexA repressor in response to DNA damage.
Step-by-step explanation:
In the process of SOS repair in bacteria, RecA protein plays a crucial role in initiating the repair of DNA when it is damaged, including in cases of thymine dimer formation due to UV radiation. Normally, RecA stimulates the self-cleavage of LexA repressor, which otherwise binds to the promoters of SOS genes and represses their transcription. When SOS is not induced, the constitutive promoter of uvrB (and other SOS genes) is repressed by LexA, preventing their expression. Upon DNA damage, activated RecA promotes the self-cleavage of LexA, leading to derepression of these genes and initiation of the SOS response.
Looking at the choices provided, the step NOT a part of the SOS repair is 'C. All targets of RecA are cleaved at the dipeptide Ala-Gly'. This is not accurate because the cleavage specifically refers to the self-cleavage of the LexA repressor induced by RecA, rather than cleaving all targets at a specific dipeptide sequence.