The Hospital Register did have his enlistment from Jefferson County, so the Jonesboro near Bessemer is most likely correct. The ADAH information is also the same. I'm sure you've seen these....

http://www.archives.state.al.us/civilwar/soldier.cfm?id=104543

Seems Benjamin G. served in the regimental hospital as a nurse / cook....

http://www.archives.state.al.us/civilwar/soldier.cfm?id=104229

http://www.archives.state.al.us/civilwar/soldier.cfm?id=104228

B.G. Johnson also originally enlisted at Jonesboro July 7, 1861. He appeared on the regimental hospital register dated same as for W.M. Johnston, attached to the hospital as nurse on Nov. 1, 1861. Return to duty January 1862. Name appears on a report dated April 25, 1862 showing names of wounded in the battle of Shiloh and those who returned from the field without arms; also those who brought in the enemy's guns in lieu of their own..guns lost 1, enemy's guns (blank), remarks (blanK). His enlistment at Auburn, Alabama was probably a re-up in 1862, and he was assigned to the regimental hospital at camp in Mobile as cook on Sept. 25, 1862.

Barry