Final answer:
Mfd is the protein in E. coli uniquely linked to transcription and DNA repair. DNA ligase is likely mutated if the joining of Okazaki fragments is impaired, and the sigma factor confers specificity to E. coli RNA polymerase during transcription. For HIV, reverse transcriptase is introduced into the host cell, and photolyase is involved in the direct repair of thymine dimers.
Step-by-step explanation:
The protein uniquely linked to transcription and DNA repair in E. coli is Mfd. This protein is responsible for transcription-coupled repair, where it recognizes RNA polymerase stalled at DNA lesions and initiates repair of the damaged DNA. The bacteria E. coli uses Mfd to couple the processes of transcription and DNA repair, ensuring that any damage to the DNA that may have potentially harmful mutations is quickly and efficiently repaired before it can be passed on during cell replication.
Regarding the subunit of the E. coli polymerase that confers specificity to transcription, it is the sigma factor (not listed in the options provided). The sigma factor is necessary for the initiation phase of transcription, as it guides the RNA polymerase to the correct promoter regions on the DNA.
In the context of the HIV virus, one of the components it brings into a cell is the enzyme reverse transcriptase, which allows the virus to reverse-transcribe its RNA genome into DNA, which can then integrate into the host cell's genome.
For DNA repair types, the breakdown of thymine dimers by the enzyme photolyase is a form of direct repair. This mechanism directly reverses DNA damage without removing the affected base from the backbone, as seen in nucleotide excision or base excision repair processes.
If the joining of Okazaki fragments is impaired, the most likely mutated enzyme at the replication fork is DNA ligase. This enzyme is responsible for sealing the nicks between Okazaki fragments on the lagging strand, ensuring the continuity of the newly synthesized DNA strand.